WO2016076940A2

WO2016076940A2 - Duplex projectile cartridge and method for assembling subsonic cartridges for use with gas-operated firearms

Info

Publication number: WO2016076940A2
Application number: PCT/US2015/047618
Authority: WO
Inventors: G. David Tubb; George Wyatt TUBB
Original assignee: Superior Shooting Systems, Inc (Tx Corp)
Priority date: 2014-08-29
Filing date: 2015-08-29
Publication date: 2016-05-19
Also published as: US20170276463A1; WO2016076940A3

Abstract

A duplex projectile cartridge system (e.g., 400) for use in a standard rifle (e.g., 10) has a front bullet (e.g., 320) and a substantially cylindrical back bullet (e.g., 440) coaxially aligned with one another. The front bullet's base is held in the cartridge case neck far enough to engage the back bullet's front surface so that the case neck simultaneously bears upon the front bullet and the back bullet. Optionally, the cartridge configuration orients the back bullet and defines one or more gas bypass lumens to allow expanding gas to initially bypass the back bullet and drive the front bullet distally to force an inter-bullet gap (e.g., "IBG" 480) upon firing.

Description

Duplex Projectile Cartridge and Method for Assembling Subsonic Cartridges for use with Gas-Operated Firearms

BACKGROUND OF THE INVENTION

Priority Claims and Cross-Reference to Related Applications:

[0001] This application claims priority to (a) commonly owned U.S. provisional patent application no. 62044175, entitled Duplex Projectile System and Method, which was filed on August 29^th, 2014, and (b) commonly owned U.S. provisional application no. 62104987, entitled Duplex Projectile Cartridge and Method for Assembling Subsonic and Supersonic Cartridges for use with Gas-Operated

Firearms, which was filed on January 19, 2015, the entire disclosures of which are incorporated herein by reference.

Field of the Invention:

[0002] The present invention relates to ammunition used in firearms and more particularly to ammunition for use with military or tactical gas-operated semiautomatic or select fire rifles and particularly ammunition intended for use in suppressor-equipped gas-operated rifles.

Discussion of the Prior Art:

[0003] Modern firearms such as rifles (e.g., 10, as shown in FigIA) make use of cartridges that include a projectile seated in a cartridge casing (as illustrated in Figs 1 B and 1 C). The casing (e.g., 150) has an internal cavity defined therein that contains a charge of rapidly combusting propellant or powder. A primer is seated in a recess formed in a rear or proximal portion or proximal of the casing. A hole in the casing places the primer in communication with the internal cavity containing the powder. The projectile is seated in the front or distal portion of the casing such that the powder is more or less sealingly contained in the casing between the primer and the projectile.

[0004] A firearm's action is used to fire the cartridge. For example, the action can include a striker that carries a firing pin. The action can be used to advance the cartridge into a firing chamber ahead of firing. While in the firing chamber, a trigger mechanism can be used to release a sear to cause the firing pin to strike the primer, causing the primer to ignite. The primer's ignition is directed to the powder, which burns within the casing. The powder burns within the casing to generate a rapidly expanding gas, which propels the projectile out of the casing and through the barrel.

[0005] Modern gas-operated semi-automatic or select fire rifles such as the M4, M16, AR-15, AR-10, XM-110 or SR-25 (e.g., rifle 10 as illustrated in Fig. 1A) are configured to shoot a variety of types ammunition which are built to specified standards. For example, the M1 18LR special ball cartridge illustrated in Fig. 1 B is designed to fire in gas-operated semi-automatic or select fire rifles and will provide a specified ballistic performance in standard rifles (e.g., AR-10 SR-25, XM-110, M14 or M21 ). Suppressors (e.g., 12, as illustrated in Fig. 1A) are frequently used by military and police marksmen and the incorporation of a suppressor into a rifle system often changes the point of impact for a given type of ammunition. The barrel and gas system for Rifle 10 are illustrated in Fig. 1 E, and when the cartridge is chambered and fired, the projectile or bullet 60 travels distally down the bore and the pressurized gas from the ignited propellant behind the bullet is communicated back into the action via a gas tube lumen 18. The addition or removal of a suppressor may change the operating properties of a gas-operated semi-automatic or select fire rifle, because more or less gas is fed back into the action, and more gas may mean more force is applied to the bolt carrier which controls bolt 20, thus changing the timing of unlocking, extraction, ejection, buffering, loading and relocking (depending on the characteristics of the rifle).

[0006] Military marksmen, police marksmen and others who are trained in using such standard or issued rifles have expectations about how these rifles will function when using issued ammunition and if those training expectations are unmet by a new rifle or ammunition offering, the new offering will almost certainly be rejected as unworkable. When the standard ammunition cycles reliably in the standard rifle and shoots with acceptable precision to a specified point of impact at a selected distance, that reliable success defines the training expectations for the standard rifle system (e.g., 10).

[0007] In certain operational environments, there is a desire for ammunition which sends more than one projectile for each cartridge fired, but the prior art attempts have all failed to meet the established training expectations for users of the standard rifles. A number of unusual ammunition configurations have been developed for use by soldiers and military marksman, and several ammunition developers have experimented with ammunition having two or more projectiles (such as the Vietnam era's "M198" duplex or "salvo" cartridge (MilSpec MIL-C-60131 , as shown in Fig 1 D), which appears to have been cancelled in 1980 as unworkable. There have been many other multi-projectile ammunition configurations, but those have all essentially required a new, non-standard firearm (such as Winchester's multi-barrel "Salvo Rifle" prototype), and so have been rejected as impracticable. Examples of patented multi-projectile prototypes are illustrated in Robinson's salvo- squeezebore ammunition (US Patent 3450050), Davies' frangible shotshell (US patent 6257147), and Eckstein's composite flechette (US patent 8640622). Many of these multi-projectile ammunition prototypes were deliberately designed to increase the "beaten zone" (or area imperiled with each shot) and so deliberately achieved a wide scattered spread among individual projectiles when firing salvos (as that term was used in military musketry). This "salvo" design goal (i.e., a large beaten zone) is very different from the precision rifle shooter's goal, which is to carefully and accurately shoot into a small aiming area, very repeatably. For these reasons, the prior art has not provided an ammunition system which reliably and precisely provides multiple hits on a small target and only on that small target.

[0008] There is a need, therefore, for a novel ammunition configuration which provides the benefits of multiple projectiles fired with each cartridge, but which does not frustrate the training expectations for users of the standard rifles (with or without suppressors), and does not create a "salvo effect" or provide a dangerously enlarged and imprecise beaten zone around the aim point or target. The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this

background is only provided to define relevant nomenclature and illustrate one exemplary technology area where some examples described herein may be practiced. SUMMARY OF THE INVENTION

[0009] The duplex projectile system and method of the present invention provides an accurate and reliable novel ammunition configuration which provides the fire superiority benefits of multiple projectiles fired with each cartridge while preserving the training expectations for users of the standard military or tactical rifles (e.g., AR-10 SR-25, M14 or M21), and which also provides precise, repeatable and accurate impacts on small targets over selected engagement ranges, meaning the precision shooter can fire two projectiles with each trigger squeeze and deliver accurate hits on small targets while avoiding unintended hits on adjacent objects or areas near the intended target.

[0010] The duplex projectile system of the present invention is optimized to provide subsonic ammunition which is adapted for use in a standard rifle (e.g., 10) equipped with a suppressor or silencer (e.g., 12). "Subsonic" in this context means ammunition which propels a projectile at a velocity intentionally selected to be below the speed of sound (e.g., below 1126 feet per second (fps) in dry air at about 70°F of slower than Mach 1 ), and subsonic ammunition is usually selected for use with a suppressor or silencer (e.g., 12) because the passing projectile won't generate the supersonic "crack" noise heard by those in the vicinity of a projectile having a velocity faster than the speed of sound (e.g., more than 1126 fps in dry air at about 70°F or faster than Mach 1 ). Briefly, that supersonic crack is created when a supersonic projectile passes through the air and creates a series of pressure waves in front of it, similar to the bow waves created by a boat. These waves travel at the speed of sound, and since the speed of the projectile is higher than those "bow" waves, the waves are forced or compressed together to create a shock wave which is audible as the supersonic crack which travels along the trajectory of the passing projectile. When a gas operated rifle (e.g. 10) is equipped with a suppressor (e.g., 12) and the shooter wants to avoid making excessive noise, the shooter will typically use subsonic ammunition which creates significantly less gas port pressure than when firing standard (supersonic) ammunition. As noted above, traditional subsonic ammunition often creates problems in that the rifle's gas system may not cycle reliably, so a shooter who doesn't want to modify the gas system of the rifle is required to shoot the louder standard supersonic ammunition. [0011] The subsonic embodiments of the duplex projectile cartridge system of the present invention create significantly more pressure at the gas port (e.g., 18) than standard single projectile subsonic cartridges, thus allowing standard rifles (e.g., 10) to function without requiring a gas system adjustment. This benefit is important because the first and second bullets of the duplex system of the present invention weigh more than any single bullet ever manufactured for use in a standard service rifle (i.e., AR-10 SR-25, M14 or M21 ). The two bullets require a higher gas pressure to reach the upper end of subsonic velocities (e.g. 1050 fps). This higher gas pressure makes it possible for a user's standard rifle to fire a subsonic load quietly and function without requiring adjustment of the gas system. No other ammunition can provide reliable gas system operation and consistent shot-to-shot subsonic accuracy in standard rifles.

[0012] Significantly, this level of performance is achieved in part because the barrels in standard rifles were discovered to provide surprising stability for the first and second projectiles in the duplex projectile system of the present invention. A standard rifle (e.g., 10) typically has a barrel (e.g., 14) with rifling having a twist rate of between one (360 degree) twist in ten inches to one in twelve inches and is designed to stabilize a single projectile of 147 grains to 175 grains travelling at the standard velocity (e.g., 2650fps-2800fps). In the present invention, the front and back bullets are each stabilized in that standard twist-rate rifle barrel (e.g., Hat subsonic velocity. The applicants have discovered how to provide the enhanced terminal ballistic benefits of firing a single long and heavy bullet at subsonic velocities, but without requiring a different barrel; a single long bullet that weighed the same amount as the duplex system's front and back bullets would be too long to be stabilized in a standard twist-rate rifle.

[0013] The duplex projectile system and method of the present invention are specifically designed for use by trained users of modern gas-operated semiautomatic or select fire standard rifles (such as the rifle type illustrated in Fig. 1A) which are configured to shoot a variety of types standard ammunition and provide a specified ballistic performance in standard (e.g., M4, AR-15, AR-10 SR-25, FN-FAL, M14 or M21 ) rifles. Suppressors are frequently used by military and police marksmen and the incorporation of a suppressor into a rifle system often changes the point of impact for a given type of ammunition. The addition or removal of a suppressor may also change the operating properties of a gas-operated semiautomatic or select fire rifle, since more gas is fed back into the action, and more gas means more force is applied to the bolt carrier, thus changing the timing of unlocking, extraction, ejection, buffering, loading and relocking (as noted above).

[0014] As also noted above, military marksmen, police marksmen and others who are trained in using such standard or issued rifles have expended significant effort at great expense and so have well established expectations about how these rifles will function when using any issued ammunition, and those hard-earned training expectations must be met by any new rifle or ammunition offering, meaning that the new ammunition must cycle reliably in the marksman's standard rifle and shoot with at least an acceptable level of precision to a specified point of impact at a selected distance.

[0015] The duplex projectile cartridge of the present invention includes a duplex bullet assembly comprising a front bullet and a back bullet which are carried in a cartridge case resembling the cartridge case for the M1 18LR (7.62 NATO) cartridge (or case 150 for the M1 18 cartridge of Figs. 1 B and 1 C). The case of the present invention is substantially cylindrical and symmetrical about a central axis 150A extending from the substantially closed proximal rimless head to the substantially open distal or neck end and the central case body defines an interior volume for receiving and containing a propellant charge. In the completed assembly, the front bullet and the back bullet are coaxially aligned with one another and with the case's central axis and are held in the case's neck by tensile force bearing upon the front bullet and the back bullet. In the exemplary embodiment, the rimless case is made of brass or steel and is manufactured to a standard

configuration (e.g., substantially identical to that used for the M1 18 cartridge shown in Figs. 1 B and 1 C).

[0016] The front bullet is preferably configured with an open tip or a pointed conical polymer ballistic tip and is fabricated or machined from one or more selected metals. The body of the front bullet has a tapered or contoured ogive terminating distally in an open tip which defines a front facing cavity or opening symmetrically defined around the central axis, and the front facing cavity may be configured to receive a polymer ballistic tip insert. The diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm) for use in a selected rifle (e.g., an FN-FAL, AR-10, SR-25, M14 or M21). The central portion of the body of the front bullet preferably includes a sidewall segment carrying a plurality of circumferential grooves of shallow depth and spaced longitudinally along the bullet sidewall from one another. The proximal or rearward portion of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a tapered sidewall segment which then transitions to a rearwardly projecting frustoconical "boat tail" which is symmetrically defined around the front bullet body's central axis, and the frustoconical boat tail terminates proximally or rearwardly in a substantially planar transverse rear end or surface configured to be received snugly within the front cavity of the back bullet.

[0017] The back bullet is preferably configured with an open tip and is fabricated or machined from one or more selected metals. The largest outer diameter or "caliber" of the back bullet is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The body of the back bullet lacks the conventional rounded or tapered nose and terminates at the front (or distally) in a very wide distal or front-facing concavity which defines a front facing opening symmetrically defined around the central axis, and that front facing concavity is precisely configured to snugly receive, center and support the rearwardly projecting frustoconical boat tail of the coaxially aligned front bullet. The central portion of the body of the back bullet preferably includes a sidewall segment carrying a plurality of circumferential grooves of shallow uniform depth but varying width and those circumferential grooves are spaced longitudinally along the back bullet sidewall from the distal or front end to the proximal or rear end. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a substantially planar transverse rear end or surface which provides a flat base back bullet configuration.

[0018] The method of assembling the duplex ammunition of this embodiment is to provide a vertically oriented standard military cartridge case which has been primed, insert a selected propellant or powder charge into the case's interior volume through the cartridge case mouth, insert a back bullet's proximal base into the cartridge case mouth and drive the back bullet down into the cartridge case's mouth so that the back bullet's distal or forward edge is recessed into the cartridge case mouth and driven 150 thousandths of an inch into the case neck, such that approximately 150 thousandths of the case neck interior is uncovered by the now inserted back bullet, when looking into the cartridge case mouth. Next a front bullet's proximal boat-tail base is inserted into the cartridge case mouth and the front bullet is driven down and seated into the back bullet's open distal end or mouth so that the front bullet's proximal boat-tail is received in and centered by the rear bullet's distal or forward surfaces when the front bullet is recessed into the cartridge case mouth and driven 150 thousandths of an inch into the case neck, so that once the duplex cartridge is assembled, the case neck supports the rear of the front bullet and the front of the rear bullet simultaneously.

[0019] In an alternative embodiment, the back bullet has a "wad cutter" configuration. In this second embodiment, the front bullet is preferably configured with a flat circular base and the body of the front bullet has a tapered or contoured ogive terminating distally in a distal solid tip or a front facing cavity or opening symmetrically defined around the central axis, and the front facing cavity may be configured to receive a polymer ballistic tip insert. The diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm) for use in a selected rifle (e.g., an FN-FAL, AR- 10, SR-25, M14 or M21 ). The central portion of the body of the front bullet preferably includes a sidewall segment carrying a plurality of circumferential grooves of shallow depth and spaced longitudinally along the bullet sidewall from one another. The proximal or rearward portion of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a transverse flat, circular base which is symmetrically defined around the front bullet body's central axis, and the substantially planar transverse rear flat base surface is configured to abut a substantially planar "wad-cutter" front surface of the back bullet.

[0020] The back "wad cutter" bullet is fabricated or machined from a selected metal or is configured with a cladding metal jacket over a lead core. The diameter or "caliber" of the back bullet is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The body of the back bullet lacks the conventional rounded or tapered nose and terminates at the front (or distally) in a full-diameter substantially planar and circular "wad-cutter" front surface symmetrically defined around the central axis, and that "wad-cutter" front surface is precisely configured to snugly abut and support the flat base of the coaxially aligned front bullet. The central portion of the body of the back bullet preferably includes a sidewall segment carrying a plurality of knurled sections or circumferential grooves of shallow uniform depth but varying width and those circumferential grooves are spaced longitudinally along the back bullet sidewall from the distal or front end to the proximal or rear end. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a substantially planar transverse rear end or surface which provides a flat base back bullet configuration.

[0021] Another (third) embodiment includes a front bullet configured with a flat circular base and the body of the front bullet has a tapered or contoured ogive terminating distally in a distal solid tip or a front facing cavity or opening

symmetrically defined around the central axis, and the front facing cavity may be configured to receive a polymer ballistic tip insert. The diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm) for use in a selected rifle (e.g., an FN-FAL, AR- 10, SR-25, M14 or M21 ). The central portion of the body of the front bullet preferably includes a sidewall segment carrying a plurality of circumferential grooves of shallow depth and spaced longitudinally along the bullet sidewall from one another. The proximal or rearward portion of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a transverse flat, circular base which is symmetrically defined around the front bullet body's central axis, and the substantially planar transverse rear flat base surface is configured to abut a substantially planar "wad-cutter" front surface of the back bullet.

[0022] Another (fourth) embodiment provides a duplex projectile system ammunition assembly and a surprisingly effective method for creating separation between the first and second bullets within the rifle's bore. Upon firing, the cartridge's ignited powder creates an expanding gas bubble which initially urges both the front and back bullets distally into the barrel's leade, where the front bullet engraves itself on the rifling and begins to accelerate both in its stabilizing rotation about the bullet's central axis and in its travel distally down the bore toward the muzzle. The front bullet is initially pushed by the back bullet. A plurality of ports or longitudinal gas-ducting grooves or channels are defined in the distal or forward portion of the back bullet to allow expanding gas flowing distally into the barrel behind the distally moving front bullet, to pressurize the base of the front bullet and force it distally down the bore while the back bullet is moving slightly more slowly, thereby creating an inter-bullet gap between the distally forced accelerating front bullet and the distal or front edge of the slower back bullet as both bullets travel distally down the bore. This inter bullet gap defines a captive or trapped volume of expanding gas between the front and back bullets as both travel distally down the bore and allows each bullet to accelerate independently. Within the barrel, each bullet is also independently spin stabilized by the rifling, so the rifling twist rate need not be optimized for a very, very long and heavy (e.g., 330 grains) and instead a standard twist rate stabilizes each bullet separately.

[0023] The above and still further features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, particularly when taken in conjunction with the accompanying drawings, wherein like reference numerals in the various figures are utilized to designate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figs 1A and 1 E illustrate the features and operation of a contemporary standard military or tactical rifle system, (e.g., AR-10, SR-25 or XM-1 10) in accordance with the prior art.

[0025] Figs 1 B and 1 C illustrate a standard ammunition assembly known officially as the M1 18LR 7.62 NATO cartridge, intended for use in many standard military or tactical rifle systems (e.g., AR-10 SR-25, XM-1 10, FN-FAL, M14 or M21 ).

[0026] Fig 1 D illustrates another prior art ammunition assembly intended for use in standard military or tactical rifle systems, specifically the "M198" duplex or "salvo" cartridge (see, e.g., MilSpec MIL-C-60131 ). [0027] Figs 2A and 2B illustrate a first embodiment of the duplex projectile system ammunition assembly and method of the present invention.

[0028] Figs 3A and 3B illustrate a second embodiment of the duplex projectile system ammunition assembly and method of the present invention.

[0029] Figs 4A, 4B and 4C illustrate the details of the front projectile configured for use in the duplex projectile system ammunition assembly and method of Figs 3A and 3B.

[0030] Figs 5A, 5B and 5C illustrate the details of an alternative, lighter front projectile configured for use in the duplex projectile system ammunition assembly and method of Figs 3A and 3B.

[0031] Figs 6A, 6B and 6C illustrate the details of the rear projectile configured for use in the duplex projectile system ammunition assembly and method of Figs 3A and 3B.

[0032] Figs 7A, 7B and 7C illustrate the details of an alternative, heavier rear projectile configured for use in the duplex projectile system ammunition assembly and method of Figs 3A and 3B.

[0033] Fig 8A and 8B illustrate, diagrammatically, plotted graphs illustrating chamber pressure (in PSI) and velocity (in fps) as a function of projectile travel distally down the bore of a rifle for two embodiments of the duplex projectile system ammunition assembly and method of the present invention and Fig 8C provides a comparable graph illustrating chamber pressure (in PSI) and velocity (in fps) as a function of projectile travel distally down the bore of a rifle for a single (well known) traditional projectile.

[0034] Figs 9A, 9B, 9C and 9D illustrate another embodiment of the duplex projectile system ammunition assembly and method of the present invention.

[0035] Figs 9E, 9F, and 9G illustrate the on-target performance for the duplex projectile system ammunition assembly and method of the present invention.

[0036] Figs 10A-E illustrate another embodiment of the duplex projectile system ammunition assembly and method of assembling "gas bypass" duplex cartridge, in accordance with the present invention.

[0037] Figs 1 1 A - 1 1 D illustrate an alternative embodiment for the duplex projectile system ammunition assembly and method for creating separation between the first and second bullets, in accordance with the present invention.

[0038] Figs 12A - 12G illustrate a preferred alternative embodiment for the duplex projectile system ammunition assembly and method for creating separation between the first and second bullets, in accordance with the present invention.

[0039] Figs 13A - 13D illustrate another alternative embodiment for the duplex projectile system ammunition assembly and method for creating separation between the first and second bullets, in accordance with the present invention.

[0040] Figs 14A and 14B illustrate another alternative embodiment for the duplex projectile system ammunition assembly and method for creating separation between the first and second bullets, in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0041] Figs 2A-14B illustrate embodiments of the duplex projectile

ammunition assembly (e.g., 100, 200, 300, 500, 600, 700 and 800) and methods for making and assembling selected components to provide a surprisingly accurate and reliable ammunition system which provides the fire superiority benefits of multiple projectiles fired with each round while preserving the hard-earned training

expectations for users of the standard military or tactical rifles (e.g., 10), and which also provides precise, repeatable and accurate impacts on small targets over selected engagement ranges. The duplex projectile system of the present invention enables the precision shooter to fire two projectiles with each trigger squeeze and deliver accurate hits on small targets (e.g., the targets with Yz inch squares shown in Figs 9E-9G) while avoiding unintended hits on adjacent objects or areas near the intended target.

[0042] Applicant's initial development work led to creation of the first embodiment of the duplex projectile system and method of the present invention, as illustrated in Figs 2A and 2B. Duplex projectile system 100 is configured for use by trained users of modern gas-operated semi-automatic or select fire standard rifles (e.g., 10) which are configured to shoot a variety of standard ammunition types and provide a specified ballistic performance in standard rifles (e.g., AR-10 SR-25, XM- 1 10, FN-FAL, M14 or M21). Suppressors (e.g., 12) are frequently used by military and police marksmen and the incorporation of a suppressor into a rifle system typically changes the point of impact for a given type of ammunition. As noted above, addition of suppressor 12 also changes the operating properties of the rifle, because more gas is fed back into the action, and more gas means more force is applied to the bolt carrier, thus changing the timing of unlocking, extraction, ejection, buffering, loading and relocking. As noted above, military marksmen, police marksmen and others who are trained in using standard issued rifles have been trained at great expense and so have built-in expectations about how these rifles will function when using any issued ammunition, and those training expectations will be met by duplex projectile system 100 and cycle reliably in the standard rifle 10 while shooting with surprisingly repeatable precision to a specified point of aim (e.g., the ½ inch squares shown in Figs 9E-9G) at a selected distance (e.g., 50 yards), as discussed below.

[0043] Referring first to Figs 2A and 2B, duplex projectile cartridge 100 includes a longitudinally, axially aligned two bullet assembly comprising a front or distal projectile or bullet 120 and a back or proximal projectile or bullet 140 which are carried in a cartridge case 150 resembling the cartridge case for the M1 18LR case of Fig. 1 B. Cartridge case 150 is substantially cylindrical and symmetrical about a central axis 150A (see Fig. 1 C) extending from the substantially closed proximal head 152 to the distal end of neck 158 which defines substantially open lumen 154 and the central case body defines an interior volume 156 for receiving, containing and protecting a propellant charge 150F (not shown). Cartridge head 152 has a substantially planar rear surface with a central primer pocket 152P which is in communication with the interior volume 156 via a flash-hole lumen, as is customary for "boxer-primed" cartridge assemblies. The cartridge neck 158 is substantially cylindrical and extends from the open distal neck end which defines neck lumen 54 rearwardly or proximally to the angled shoulder 150S which flares out to the substantially cylindrical body sidewall 150SW, and the axial length of the cartridge neck 158 is preferably 0.310 inches and the axial length of the front and back bullets 120, 140 is 2.260 inches, for the illustrated example.

[0044] Front bullet 120 and the back bullet 140 are coaxially aligned with one another and with the case's central axis 150A and are held in case neck 158 by inwardly squeezing circumferential force (or "neck tension") applied via the case neck 158 simultaneously bearing upon and supporting front bullet 120 and back bullet 140, as shown in Figs 2A and 2B. In the illustrated embodiment, rimless case 150 is substantially identical to the case used for the M118LR cartridge illustrated in Fig. 1 B but other cartridge cases manufactured in conformance with 7.62 NATO ammunition specifications (e.g., the M80 7.62 NATO case) may be used in this exemplary embodiment. For ammunition in different calibers (e.g., SS109/M855 5.56 NATO cartridges or A191/MK 248 .300 Winchester magnum cartridges), cartridge cases manufactured in conformance with those respective ammunition specifications can be adapted for use in the duplex ammunition system and method of present invention.

[0045] Front bullet 120 is preferably configured with an open tip 122 or a pointed conical metal alloy or polymer ballistic tip (not shown) and is fabricated or machined from one or more selected metals (e.g., lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass). The body of the front bullet has a tapered or contoured ogive terminating distally in open tip 122 which defines a front facing cavity or opening symmetrically defined around the bullet's central axis, and the front facing cavity may be configured to receive a metal alloy or polymer ballistic tip insert (not shown). The diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm) for use in a selected rifle (e.g., 10). The central portion of the body of the front bullet preferably includes a sidewall segment carrying a plurality of distally projecting radial sidewall segments separated by circumferential grooves of shallow depth (e.g., 14-24 one thousandths of an inch) and spaced longitudinally along the bullet sidewall from one another. The proximal or rearward portion of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a tapered sidewall segment which then transitions to a rearwardly projecting frustoconical boat tail 126 which is symmetrically defined around the front bullet body's central axis, and the frustoconical boat tail terminates proximally or rearwardly in a substantially planar transverse rear end or surface 128 configured to be received snugly within a front cavity of back bullet 140.

[0046] Back bullet 140 is preferably configured with a distal open tip 142 and is fabricated or machined from one or more selected metals (e.g., lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass). The diameter or "caliber" of back bullet 140 is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The body of back bullet 140 lacks the conventional rounded or tapered ogive or nose and terminates at the front (or distally) in a very wide distal or front-facing concavity which defines a front facing opening 142 symmetrically defined around the bullet's central axis, and that front facing concavity 142 is precisely configured to snugly receive, center and support the rearwardly projecting frustoconical boat tail 126 of the coaxially aligned front bullet 120.

[0047] The central portion of the body of the back bullet preferably includes a sidewall segment carrying a plurality of distally projecting radial sidewall segments separated by circumferential grooves of shallow uniform depth (e.g., 14-24 one thousandths of an inch) but varying width and those circumferential grooves are spaced longitudinally along the back bullet sidewall from the distal or front end to the proximal or rear end. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a substantially planar transverse rear end or base surface 146 which provides a flat- base configuration for back bullet 140. When front bullet boat tail 126 is seated within back bullet cavity 142, there is a tapered annular gap of about 0.005 inches separating the outer diameter surface of front bullet boat tail 126 from the interior surface of back bullet cavity 142, as best seen in the enlarged detail view of Fig. 2B.

[0048] Alternative embodiments of the duplex cartridge system (e.g., 200) and alternatives for the front bullet (e.g., 220, 220A) and back bullet (e.g., 240, 240A) are illustrated in Figs 3A-7C. Referring specifically to Figs 3A and 3B, duplex projectile cartridge 200 includes a longitudinally, axially aligned two bullet assembly

comprising front bullet 220 and a back bullet 240 which also have a combined axial length of 2.260 inches as carried in a cartridge case 150. Front bullet 220 and back bullet 240 are coaxially aligned with one another and with the case's central axis and are held in case neck 158 by inwardly squeezing circumferential tensile force (or "neck tension") applied via the case neck simultaneously bearing upon and supporting front bullet 220 and back bullet 240, as shown in Figs 3A and 3B. In the illustrated embodiment, rimless case 150 is substantially identical to the case used for the M1 18 cartridge illustrated in Fig. 1 B, but other cartridge cases manufactured in conformance with 7.62 NATO ammunition specifications may be used. As in the prior embodiment, front bullet 220 is preferably configured with a hexagonal-section open tip 222 or a pointed conical metal alloy or polymer ballistic tip (not shown) and is fabricated or machined from one or more selected metals (e.g., lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass) preferably providing a projectile weight of 175.7 grains.

[0049] Referring now to Figs 4A-4C, the body of the front bullet 220 has a tapered or contoured ogive terminating distally in open tip 222 and the diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm). The length of front bullet 220 is preferably 1.3 inches from the front or distal end (at opening 222) to base surface 228 (or about 4.22 times the caliber or largest diameter) with the other dimensions as shown in Fig. s 4B and 4C. The central portion of the body of the front bullet preferably includes a sidewall segment carrying a plurality of distally projecting radial sidewall segments separated by circumferential grooves of shallow depth (e.g., 6-24 one thousandths of an inch) and spaced longitudinally along the bullet sidewall from one another. The proximal or rearward portion of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a tapered sidewall segment which then transitions to a rearwardly projecting frustoconical boat tail 226 which is symmetrically defined around the front bullet body's central axis, and the frustoconical boat tail 226 terminates proximally or rearwardly in a

substantially planar transverse rear end or surface 228 configured to be received snugly within front cavity 242 of back bullet 240.

[0050] Back bullet 240 (see Figs 6A-6C and Figs 3A and 3B) is preferably configured with a distal open tip 242 and is fabricated or machined from one or more selected metals (e.g., lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass) to provide a preferred projectile weight of 161.7 grains. The diameter or "caliber" of back bullet 240 is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The length of back bullet 240 is preferably 1.3 inches from distal end (at 242) to base surface 246 (or about 4.22 times the caliber or largest diameter) with the other dimensions as shown in Figs 6B and 6C. The body of back bullet 240 lacks the conventional rounded or tapered ogive or nose and terminates at the front (or distally) in a very wide distal or front-facing concavity which defines a front facing opening or cavity 242 symmetrically defined around the bullet's central axis, and that front facing concavity 242 is precisely configured to snugly receive, center and support the rearwardly projecting frustoconical boat tail 226 of the coaxially aligned front bullet 220.

[0051] The central portion of the body of the back bullet preferably includes a sidewall segment carrying a plurality of distally projecting radial sidewall segments separated by circumferential grooves of shallow uniform depth (e.g., 14-24 one thousandths of an inch) but varying width and those circumferential grooves are spaced longitudinally along the back bullet sidewall from the distal or front end to the proximal or rear end 246. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a substantially planar transverse rear end or base surface 246 which provides a flat- base configuration for back bullet 240. When front bullet boat tail 226 is seated within back bullet cavity 242, there is a tapered annular gap of about 0.005 inches separating the outer diameter surface of front bullet boat tail 226 from the interior surface of back bullet cavity 242, as best seen in the enlarged detail view of Fig. 3B.

[0052] Another embodiment of the duplex cartridge system (e.g., 200) provides alternatives for the front bullet (e.g., 220A) and back bullet (e.g., 240A) as illustrated in Figs 5A-5C and 7A-7C. Referring specifically to Figs 5A-5C, the body of the front bullet 220A has a tapered or contoured ogive terminating distally in open tip 222A and the diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm), while providing a lighter projectile weight of, preferably, 135.8 grains, due to fabrication from lighter metal(s) than the front bullet embodiments described above. The length of front bullet 220A is preferably 1 .3 inches from distal end (at opening 222A) to base surface 228A (or about 4.22 times the caliber or largest diameter) with the other dimensions as shown in Figs 5B and 5C.

[0053] The central portion of the body of the front bullet preferably includes a sidewall segment carrying a plurality of distally projecting radial sidewall segments separated by circumferential grooves of shallow depth (e.g., 6-24 one thousandths of an inch) and spaced longitudinally along the bullet sidewall from one another. The proximal or rearward portion of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a tapered sidewall segment which then transitions to a rearwardly projecting frustoconical boat tail 226A which is symmetrically defined around the front bullet body's central axis, and the

frustoconical boat tail 226A terminates proximally or rearwardly in a substantially planar transverse rear end or surface 228A configured to be received snugly within front cavity 242 of back bullet 240B.

[0054] Back bullet 240B (see Figs 7A-7C) is preferably configured with a distal open tip 242B and is fabricated or machined from one or more selected metals (e.g., lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass) to provide a heavier projectile weight of 209.3 grains. The diameter or "caliber" of back bullet 240 is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The length of back bullet 240B is preferably 1.3 inches from distal end (at opening 242) to base surface 246 (or about 4.22 times the caliber or largest diameter) with the other dimensions as shown in Figs 7B and 7C. The body of back bullet 240B lacks the conventional rounded or tapered ogive or nose and terminates at the front (or distally) in a very wide distal or front-facing concavity which defines a front facing opening or cavity 242B symmetrically defined around the bullet's central axis, and that front facing concavity 242B is precisely configured to snugly receive, center and support the rearwardly projecting frustoconical boat tail 226A of the coaxially aligned front bullet 220A. The central portion of the body of the back bullet 240B preferably includes a sidewall segment carrying a plurality of distally projecting radial sidewall segments separated by circumferential grooves of shallow uniform depth (e.g., 14- 24 one thousandths of an inch) but varying width and those circumferential grooves are spaced longitudinally along the back bullet sidewall from the distal or front end to the proximal or rear end 246B. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a substantially planar transverse rear end or base surface 246B which provides a flat-base configuration for back bullet 240B. When front bullet boat tail 226A is seated within back bullet cavity 242B, there is a tapered annular gap of about 0.005 inches separating the outer diameter surface of front bullet boat tail 226 from the interior surface of back bullet cavity 242, as with the embodiment seen in the enlarged detail view of Fig. 3B.

[0055] The method of assembling the duplex cartridge system (e.g., 100 or 200) is to provide a vertically oriented standard military cartridge case 150 which has been primed, insert a selected propellant or powder charge (e.g., 160 or 260, such as Hodgdon H4350 or H1000) into the case's interior volume through the cartridge case mouth lumen (e.g., 154), insert a back bullet (e.g., 140, 240, 240B) with the back bullet's proximal base into the cartridge case mouth 154 and drive the back bullet axially and proximally down into the cartridge case's mouth so that the back bullet's distal or forward edge is recessed into the cartridge case mouth and driven substantially half way down the neck, or one hundred fifty thousandths of an inch into the case neck158 , such that approximately half (or one hundred fifty

thousandths) of the case neck lumen's interior sidewall is uncovered by the now inserted back bullet, when looking into the cartridge case mouth. Next a front bullet (e.g., 120, 220 or 220A) proximal boat-tail base is inserted into the cartridge case mouth 154 and the front bullet is driven axially down or proximally and seated into the back bullet's open distal end or mouth (e.g., 142) so that the front bullet's proximal boat-tail (e.g., 126) is received in and centered by the rear bullet's distal or forward surfaces when the front bullet is recessed into the cartridge case mouth and driven one hundred fifty thousandths of an inch into the case neck, so that once the duplex cartridge system (e.g., 100 or 200) is assembled, the case neck 158 supports the rear of the front bullet and the front of the rear bullet simultaneously.

[0056] Testing of prototype duplex cartridge systems (e.g., 100 or 200) of the present invention for embodiments intended to generate projectile velocities that are substantially subsonic or transonic was confirmed with chronometer testing to confirm muzzle velocities (e.g., the shots fired for the target shown in Fig. 9G were measured at 040 FPS). Computer modelling was used to confirm that the duplex ammunition assembly of the present invention would always generate a more than adequate amount of port pressure to cycle a standard rifle's action (where standard rifles (e.g., 10) typically require at least 10,500 PSI. An exemplary Military Specification gas port pressure requirement is 12,500psi +/- 2000psi (See, e.g., Mil- C-46934B for the M118 cartridge which specifies gas port pressure for the M14 rifle). The Pressure-Velocity graphs illustrated in Figs. 8A and 8B show velocity (of the front or first projectile) in the upper trace and pressure in the lower trace. The pressure (lower) trace in Fig. 8A reflect chamber (and thus port) pressure for a 295 grain duplex bullet assembly (e.g., 100, 200) fired in a .308 Winchester SAAMI specification chamber (e.g., in rifle 10) where the casing is charged with 19.8 grains of smokeless powder propellant (e.g., 160 or 260, Hodgdon™ H4350). For the example modelled in Fig 8A, the front and back bullets were seated into the neck to provide overall length ("OAL") of 2.80 inches. This load, as modelled, generated chamber pressure data which illustrates that the chamber pressure is initially over 15,000PSI and remains over 10,000 PSI as the duplex projectiles travel down the bore (e.g., of rifle 10). Fig. 8B illustrates pressure vs. projectile travel for duplex assembly 600 with a combined weight of 350 grains (as seen in Figs 12A-12G), where the casing is charged with 23 grains of smokeless powder propellant (e.g., Hodgdon H1000). Fig. 8C is provided here for comparison, illustrating pressure vs. travel for a single 240 gr. Sierra™ subsonic projectile (i.e., a 220 grain Sierra Match King™ bullet) for comparison.

[0057] As noted above, to meet the normal (supersonic) specification, the ammunition needs to supply 10,500psi at the gas port to reliably cycle the action. Referring back to Fig. 8B, the pressure ported back into the action depends on the length of barrel between the chamber and the port. "AR" style rifles (e.g., 10) can have different length gas systems, so, as shown in Fig. 8B, for a 350 gr Duplex cartridge assembly (e.g., 600) the Gas Port Length vs. Chamber Pressure

observations are: (a) Carbine = 7in. = 10,500 psi, (b) Mid-length = 9in = 8,800 psi and (c) Rifle = 12in = 7, 100 psi. In applicant's trials, each of these embodiments worked, meaning that a standard rifle configured as rifle 10 is configured generated adequate port pressure to reliably cycle. Turning to Fig. 8C, for comparison, a 220 grain Sierra single projectile load provides unreliable cycling, especially for a rifle having a standard length gas system, due in part to Gas Port Length vs. Chamber Pressure observations, which are: (a) Carbine = 7in. = 7, 100 psi, (b) Mid-length = 9in = 6,000 psi, and (c) Rifle = 12in = 5,000 psi. As a quick method for evaluating the internal ballistics of these ammunition varieties in various rifles (e.g., 10), we estimated that the chamber pressure is nearly equal to the gas port pressure as the projectile passes the gas port lumen (so that the gas port (e.g., 18) is pressurized. Duplex ammunition (e.g., 600) theoretically meets the Mil Spec Requirement with a 7 in. long gas port, also other gas port lengths are close to the required port pressure. Comparing the pressures from 350 gr. Duplex load (e.g., 600, shown in Fig. ) vs. the 220 gr Sierra single bullet load, on average the duplex ammunition supplies about 45% more pressure to the gas port 18. This is significantly more pressure, and this difference in pressure at the gas port 18 explains why the subsonic duplex ammunition assembly of the present invention (e.g., 100, 200, 300, 400, 500, 600, 700, 800) is able to cycle many standard semi-automatic firearms (e.g., 10) without any modification to the firearm. It is understood that the gas port pressure will differ slightly from the chamber pressure as plotted in Figs 8A-8C, but applicant's development work and test results from several trials indicate that the duplex bullet cartridge assemblies of the present invention do cycle reliably in standard issue rifles.

[0058] An alternative embodiment is illustrated in Figs 9A-9D, where duplex cartridge system 300 includes a front bullet 320 which is preferably configured as a spitzer with a flat circular base and the body of front bullet 320 has a tapered or contoured ogive terminating distally in a distal solid tip or a front facing open tip or cavity or opening symmetrically defined around the central axis, and the front facing cavity may be configured to receive a polymer ballistic tip insert (not shown). The diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm) for use in a selected rifle (e.g., 10). The central portion of the body of front bullet 320 may optionally include a sidewall segment carrying a plurality of circumferential grooves of shallow depth and spaced longitudinally along the bullet sidewall from one another or a knurled segment. The proximal or rearward portion 326 of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a transverse flat, circular base 328 which is symmetrically defined around the front bullet body's central axis, and the substantially planar transverse rear flat base surface 328 is configured to abut a substantially planar "wad-cutter" front surface of the back bullet 340, although an inter-bullet gap may be defined therebetween or filled with an optional wadding disc (not shown) of selected axial thickness (e.g., 1 mm).

[0059] Front bullet 320 and back bullet 340 are coaxially aligned with one another and with the case's central axis 150A and are held in case neck 158 by inwardly squeezing circumferential force (or "neck tension") applied via the case neck 58 simultaneously bearing upon and supporting front bullet 320 and back bullet 340, as shown in Figs 9A-C. In the illustrated embodiment for duplex cartridge system 300, as with the prior embodiments, rimless case 150 is substantially identical to the case used for the M 8LR cartridge illustrated in Fig. 1 B but other cartridge cases manufactured in conformance with 7.62 NATO ammunition specifications (e.g., the M80 7.62 NATO case) may be used in this exemplary embodiment. For ammunition in different calibers (e.g., SS109/M855 5.56 NATO cartridges or A191/MK 248 .300 Winchester magnum cartridges), cartridge cases manufactured in conformance with those respective ammunition specifications can be adapted for use in the duplex ammunition system and method of present invention.

[0060] Back bullet 340 is fabricated or machined from a selected metal or is configured with a cladding metal jacket over a lead core. The diameter or "caliber" of the back bullet is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The body of back bullet 340 lacks a conventional rounded or tapered nose and terminates at the front (or distally) in a full-diameter substantially planar "wad-cutter" front surface 342 symmetrically defined around the bullet's central axis, and that "wad-cutter" front surface 342 is preferably configured to snugly abut and support the flat base 328 of the coaxially aligned front bullet 320 , although an inter-bullet gap (see Fig. 9C) may be defined therebetween or filled with an optional wadding disc (not shown). The central portion of the body of the back bullet 340 optionally includes a sidewall segment carrying a plurality of knurled sections or circumferential grooves of shallow uniform depth but varying width and those circumferential grooves are spaced longitudinally along the back bullet sidewall from the distal or front end 342 to the proximal or rear end. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a substantially planar transverse rear end or surface which provides a flat base. In the illustrated embodiment of duplex cartridge system 300, rimless case 150 is substantially identical to the case used for the M1 18LR cartridge illustrated in Fig. 1 B, but other cartridge cases manufactured in conformance with 7.62 NATO ammunition specifications (e.g., the M80 7.62 NATO case) may be used in this exemplary embodiment. For ammunition in different calibers (e.g., SS109/M855 5.56 NATO cartridges or A191/MK 248 .300 Winchester magnum cartridges), cartridge cases manufactured in conformance with those respective ammunition specifications can be adapted for use in the duplex ammunition system and method of present invention.

[0061] The method of assembling the duplex ammunition 300 of Figs 9A-9D is to provide a vertically oriented standard military cartridge case 150 which has been primed, insert a selected propellant or powder charge 360 (e.g., Hodgdon H4350 smokeless powder) into the case's interior volume through the cartridge case mouth 154, insert a back bullet's proximal base into the cartridge case mouth and drive the back bullet 340 down into the cartridge case's mouth so that the back bullet's distal or forward substantially planar "wad-cutter" front surface 342 is recessed into the cartridge case mouth and driven 150 thousandths of an inch into the case neck, such that approximatelyl 50 thousandths of the case neck interior is uncovered by the now inserted back bullet 340, when looking into the cartridge case mouth. Optionally, an inter-bullet gap defining wadding may be placed upon the distal surface of back bullet 340. Next a front bullet's proximal flat base 328 is inserted into the cartridge case mouth and the front bullet is driven down and preferably seated into abutment with either the inter-bullet gap defining wadding or with the back bullet's substantially planar "wad-cutter" front surface 342 so that the front bullet's proximal flat base 328 is received against the rear bullet's substantially planar "wad-cutter" front surface 342 when the front bullet 320 is recessed into the cartridge case mouth and driven 150 thousandths of an inch into the case neck. Once the duplex cartridge 300 is assembled, the case neck 158 supports the rear of the front bullet and the front of the rear bullet simultaneously. Figs 9B, C and D are photographs of an altered example of duplex cartridge system 300, where the side of cartridge case 150 has been cut away to reveal the internal configuration of these components, and these photographs illustrate that some of the propellant or powder charge 360 surrounds back bullet 340 (best seen in Fig. 9D).

[0062] Experiments with prototypes of the duplex cartridge system of the present invention suggest that an alternative configuration for the back bullet is useful in creating separation between the front bullet (e.g., 320) and the back bullet (e.g., 440) in the rifle's barrel (e.g., 14). Surprising test firing results are shown on the target images of Figs 9E-9G, where, in each case, five rounds of the subsonic duplex cartridge system of the present invention were fired at a target 50 yards away, aiming particularly at the targets' ^ΛΛ inch squares with a suppressed, standard rifle (e.g., 10). In the test target of Fig. 9E, five shots or rounds were fired and the first shot's impacts are designated as "1 ", for the front bullet (a spritzer, e.g., 320) and "A" for the back bullet (e.g., wadcutter 340, which makes the distinctive circular holes). The remaining four shots produced front bullet impacts 2-5 and rear bullet impacts B-E, and the composite group defined by all 10 projectile strikes illustrated in Fig. 9E fit within a 3 inch circle, meaning that at 50 yards (or about 50 meters), all ten bullet impacts from five shots group within a six Minute of Angle (MOA) circle, and within 3 MOA of the aim point (i.e., the ½ inch square). The test firings for similar targets at the same range with the same rifle illustrated in Figs 9F and 9G are similar, indicating repeatable, reliable ballistic performance.

[0063] Another embodiment of the projectile assembly 400 and method are illustrated in Figs 10A-E. A back bullet 440 initially resembling cylindrical projectile 340 has been swaged or modified to have a substantially squared cross section at the front or distal wadcutter end 442 (shown in Figs. 10B, 10D and 10E). The distal swaged end 442 of back bullet 440 creates four substantially planar angled sidewall surface segments 460, 462, 464, 466 which taper inwardly toward central axis 450A at a taper angle 470 of 4-8 degrees, terminating distally at distal surface 442 and those angled sidewall surface segments define four "gas bypass" features at the distal end of back bullet 440, as described below, The four planar surfaces are swaged into the bullet's sidewall and each tapers from a transition point 472, where the sidewall of back bullet 440 is substantially cylindrical from the four radially arrayed transition points to the proximal end or base 446. The axial length of bullet 440 from transition point 472 to the distal squared wadcutter end 442 is 0.3 to 0.6 inches for a bullet 440 having an overall axial length of 1.3 inches.

[0064] Referring specifically to Figs 10A-C, duplex projectile cartridge 400 includes a longitudinally, axially aligned two bullet assembly comprising front bullet 320 and a back bullet 440 which also preferably have a combined axial length of 2.260 inches as carried in a cartridge case 150. Front bullet 320 and back bullet 440 are coaxially aligned with one another and with the case's central axis 150A and are held in case neck 158 by inwardly squeezing circumferential force (or "neck tension") applied via the case neck simultaneously bearing upon and supporting front bullet 320 and back bullet 440, as shown in Fig 10A. In the illustrated embodiment, rimless case 150 is substantially identical to the case used for the M1 18 cartridge illustrated in Fig. 1 B, but other cartridge cases manufactured in conformance with 7.62 NATO ammunition specifications may be used. As in the prior embodiment, front bullet 320 is preferably fabricated or machined from one or more selected metals (e.g., lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass) preferably providing a projectile weight of 150 grains.

[0065] Back bullet 440 (see Figs 10B and 10C) is preferably configured with a substantially planar distal surface 442 and is fabricated or machined from one or more selected metals (e.g., lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass) to provide a preferred projectile weight of 200 grains. The diameter or "caliber" of back bullet 440 is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The length of back bullet 240 is preferably .3 inches from distal end (at 442) to base surface 446 (or about 4.22 times the caliber or largest diameter). The body of back bullet 440 lacks the conventional rounded or tapered ogive or nose and terminates at the front (or distally) in the square shaped distal front-facing surface 442 which is transverse to and symmetrically defined around the bullet's central axis 450A, and that front facing surface 442 is precisely configured to snugly abut and support the proximal flat base 328 of coaxially aligned front bullet 320. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a substantially planar transverse rear end or base surface 446 which provides a flat-base configuration for back bullet 440.

[0066] The method of assembling the duplex ammunition 400 of Fig 10A is illustrated in Figs 10D and 10E. Once a standard military cartridge case 50 has been primed (not shown), a selected propellant or powder charge 460 (e.g.,

Hodgdon H4350 or H1000 smokeless powder) is deposited into the case's interior volume through the cartridge case mouth 154 (see Fig. 10D). Next, proximal base 446 of back bullet 440 is placed over case mouth 154 and back bullet 440 is pressed axially and proximally along axis 150A into the cartridge case mouth so that back bullet 440 is forced into the cartridge case's mouth so that the back bullet's distal or forward substantially planar "wad-cutter" front surface 442 is recessed into the cartridge case mouth 154 and driven 150 thousandths of an inch into the case neck 158, such that approximatelyl 50 thousandths of the case neck interior is uncovered by the now inserted back bullet 440, when looking into the cartridge case mouth, as best seen in Fig. 10E. Next, a front bullet's proximal flat base 328 is inserted into the cartridge case mouth 154 and front bullet 320 is driven proximally down and preferably seated into abutment with the back bullet's substantially planar "wad- cutter" front surface 442 so that the front bullet's proximal flat base 328 is received against the rear bullet's substantially planar "wad-cutter" front surface 442 when the front bullet 320 is recessed into the cartridge case mouth and driven 150

thousandths of an inch into the case neck 158. Once the duplex cartridge 400 is assembled, the case neck 158 supports the rear of the front bullet and the front of the rear bullet simultaneously, and (as shown in Fig. 10E) four gas bypass lumens are provided to vent ignition gas toward the base 328 of front bullet 320.

[0067] Fig. 10E and Fig. 10A show how the tapered planar wall segments 460, 462, 464, 466 define four symmetrical gas bypass lumens between the tapered planar distal sidewall surfaces of bullet 440 and the cylindrical surface of the interior of cartridge neck 158, and these four symmetrical gas bypass lumens are configured and located to allow an optimal amount of propellant gas to bypass rear projectile 440 when the propellant is ignited by the primer (not shown).

[0068] Referring now to Fig. 10F, when the propellant charge 460 within case 150 ignites, the back bullet 440 is pushed distally and out of the case mouth 154 but while the back bullet 440 is still within case 150, the distal squared sidewall ends provide four momentary lumens or passages (two of which are seen in Fig, 10E) for the expanding gas from the burning propellant 460 which pressurizes the space behind the front bullet (e.g., 320) and drives it distally down barrel 14 to create an inter-bullet gap "IBG" 480 between the front bullet 320 and the rear bullet 440.

[0069] Gas bypass duplex projectile assembly 400 provides a surprisingly effective method for creating the inter bullet gap or bore axis longitudinal separation "IBG" 480 between the first bullet 320 and second bullet 440 within the bore (e.g., of barrel 14, as seen in Fig. 10F). Upon firing, the cartridge's ignited powder 460 creates an expanding gas bubble which initially urges both the front and back bullets distally into the barrel's leade (not shown), where front bullet 320 engraves itself on the rifling and begins to accelerate both in its stabilizing rotation about the bullet's central axis and in its travel distally down the bore toward the muzzle. Front bullet 320 is pushed by back bullet 440 and by the bypassing gas from the four bypassing lumens. The four gas bypass lumens or gas-ducting channels are defined in the distal or forward surface back bullet 440 to allow expanding gas flowing distally into the barrel (e.g., 14) behind the distally moving front bullet 320, to pressurize the base 328 of the front bullet and force it distally down the bore while back bullet 440 is moving slightly more slowly, thereby creating the inter-bullet gap "IBG" 480 between the distally forced accelerating front bullet 320 and the distal or front edge 442 of the slower back bullet 440 as both bullets travel distally down the bore. This inter bullet gap "IBG" 480 defines a captive or trapped volume of expanding gas between the front and back bullets as both travel distally down the bore and allows each bullet to accelerate and engage the barrel's rifling independently of one another. Within the barrel (e.g., 14), each bullet 320, 440 is also independently spin stabilized by the rifling (e.g., standard right hand twist, 1 twist in 12 inches, not shown), so the barrel's rifling twist rate need not be optimized for a very, very long and heavy (e.g., 350 grains) single bullet (or abutting front and rear bullets which spin and act as one long 350 gr projectile) and instead a standard (e.g., 1 :12) twist rate stabilizes front bullet 320 separately and independently from rear bullet 440, as seen in Fig. 10F. [0070] Turning now to Figs 1 1A - 14B, four alternative embodiments for the duplex projectile system ammunition assembly (500, 600, 700, and 800) are illustrated, where each embodiment provides an alternative configuration and method for creating an inter-bullet gap "IBG" 480 or longitudinal separation between the first and second bullets (as seen in Fig. 10F), in accordance with the present invention.

[0071] Figs 1 1 A-1 1 D illustrate front, rear, and cross sectional side views of a ducted gas bypass duplex projectile system ammunition assembly 500, where front bullet 320 is preferably configured as a spitzer with a flat circular base and the body of front bullet 320 has a tapered or contoured ogive terminating distally in a distal solid tip or a front facing cavity or opening symmetrically defined around the central axis, and the front facing cavity may be configured to receive a polymer ballistic tip insert (not shown). The diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm) for use in a selected rifle (e.g., 10). The central portion of the body of front bullet 320 may optionally include a sidewall segment carrying a plurality of circumferential grooves of shallow depth and spaced longitudinally along the bullet sidewall from one another or a knurled segment (not shown). The proximal or rearward portion 326 of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a transverse flat, circular base 328 which is symmetrically defined around the front bullet body's central axis, and the substantially planar transverse rear flat base surface 328 is configured to abut a substantially planar ported or ducted front surface 542 defined in back bullet 540, although an inter-bullet starter gap may be defined therebetween or filled with an optional wadding disc (not shown).

[0072] Back bullet 540 is fabricated or machined from a selected metal or is configured with a cladding metal jacket over a lead core. The diameter or "caliber" of the back bullet is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The body of back bullet 540 lacks a conventional rounded or tapered nose and terminates at the front (or distally) in a full-diameter substantially planar "wad-cutter" front surface 542 symmetrically defined around the bullet's central axis, and that "wad-cutter" front surface 542 is preferably configured to snugly abut and support the flat base 328 of the coaxially aligned front bullet 320, although an inter-bullet gap (as in Fig. 9C) may be defined therebetween or filled with an optional wadding disc (not shown). The central portion of the body of the back bullet 540 preferably includes a sidewall segment carrying a plurality of transverse apertures, lumens or ports 544 of small inside diameter (e.g., 0.05 inch ID) which are also in fluid communication with a longitudinal or axial lumen or longitudinal axial duct 545 which permits expanding gas from ignited propellant to pass distally through the solid interior of back bullet 540 to pressurize the space behind front bullet 320, when fired. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a tapered sidewall with a reduced diameter substantially planar transverse rear end to provide a boat-tail back bullet configuration.

[0073] The method of assembling the duplex ammunition 500 of Figs 1 1A-D is similar to that described above and illustrated in Figs 10D and 10E. Once a standard military cartridge case 150 has been primed (not shown), a selected propellant or powder charge 460 (e.g., Hodgdon H4350 or H1000 smokeless powder) is deposited into the case's interior volume through the cartridge case mouth. Next, proximal base of back bullet 540 is placed over case mouth 154 and back bullet 540 is pressed axially and proximally along axis 150A into the cartridge case mouth so that back bullet 540 is forced into the cartridge case's mouth and the back bullet's distal or forward substantially planar "wad-cutter" front surface 542 is recessed into the cartridge case mouth 154 and driven 150 thousandths of an inch into the case neck 158 such that approximately150 thousandths of the case neck interior is uncovered by the now inserted back bullet 540. Next, a front bullet's proximal flat base 328 is inserted into the cartridge case mouth 154 and front bullet 320 is driven proximally down and preferably seated into abutment with the back bullet's substantially planar "wad-cutter" front surface so that the front bullet's proximal flat base 328 is received against the rear bullet's substantially planar "wad- cutter" front surface when the front bullet 320 is recessed into the cartridge case mouth and driven 150 thousandths of an inch into the case neck 158. Once the duplex cartridge 500 is assembled, the case neck 158 supports the rear of the front bullet and the front of the rear bullet simultaneously, and (as shown in Fig. 1 1 B) the gas bypass lumens 544, 545 are provided to vent ignition gas toward the base 328 of front bullet 320.

[0074] Figs. 1 1 B and 1 1 D show how the gas bypass lumens within back bullet 540 are configured and located to allow an optimal amount of propellant gas to bypass rear projectile 540 when the propellant is ignited by the primer (not shown). When the primer ignites the propellant charge 460, the back bullet 540 is pushed distally and out of the case mouth 154 but while the back bullet is still within case 150, the bypass lumens 544, 545 provide momentary passages for the expanding gas from the burning powder 460 which pressurizes the space behind the front bullet (e.g., 320) and drives it down the barrel (e.g., 14) to create an inter-bullet gap (e.g., "IBG" 480) between the front bullet 320 and the rear bullet 540.

[0075] Gas bypass duplex projectile assembly 500 also provides a

surprisingly effective method for creating separation (e.g., "IBG" 480) between the first bullet 320 and second bullet 540 within the bore (e.g., of rifle 10). Upon firing, the cartridge's ignited powder 460 creates an expanding gas bubble which initially urges both the front and back bullets distally into the barrel's leade (not shown), where front bullet 320 engraves itself on the rifling and begins to accelerate both in its stabilizing rotation about the bullet's central axis and in its travel distally down the bore toward the muzzle. Front bullet 320 is pushed by back bullet 540 and by bypassing expanding gas from lumen 545. The gas bypass lumens or gas-ducting channels 544, 545 are in fluid communication with the distal or forward surface of back bullet 540 to direct expanding gas flowing distally into the barrel behind the distally moving front bullet 320 to pressurize the base 328 of the front bullet and force it distally down the bore while back bullet 540 is moving slightly more slowly, thereby creating the inter-bullet gap (e.g., "IBG" 480) between the distally forced accelerating front bullet 320 and the distal or front edge 542 of the slower back bullet as both bullets travel distally down the bore. This inter bullet gap (e.g., "IBG" 480) defines a captive or trapped volume of expanding gas between the front and back bullets as both travel distally down the bore and allows each bullet to accelerate and engage the barrel's rifling independently. Within the barrel (e.g., of rifle 10), each bullet 320, 540 is also independently spin stabilized by the rifling, so the rifling twist rate need not be optimized for a very, very long and heavy (e.g., 350 grains) single bullet (or abutting bullets which spin and act as one) and instead a standard twist rate stabilizes front bullet 320 separately and independently from rear bullet 540.

[0076] Figs 12A - 12G illustrate another alternative embodiment for the duplex projectile system ammunition assembly 600 where front bullet 320 is preferably configured as a spitzer with a flat circular base and the body of front bullet 320 has a tapered or contoured ogive terminating distally in a distal solid tip or a front facing cavity or opening symmetrically defined around the central axis, and the front facing cavity may be configured to receive a polymer ballistic tip insert (not shown). The diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm) for use in a selected rifle (e.g., 10). The central portion of the body of front bullet 320 may optionally include a sidewall segment carrying a plurality of circumferential grooves of shallow depth and spaced longitudinally along the bullet sidewall from one another or a knurled segment (not shown). Alternatively, a soft-point front bullet such as the Sierra Game King™ (e.g., 320SP) may be substituted. The proximal or rearward portion 326 of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a transverse flat, circular base 328 which is symmetrically defined around the front bullet body's central axis, and the substantially planar transverse rear flat base surface 328 is configured to abut a substantially planar transverse front surface or wide meplat 642 of the tapered-ogive back bullet 640, although an inter-bullet starter gap may be defined therebetween or filled with an optional wadding disc (not shown).

[0077] Tapered Ogive back bullet 640 is fabricated or machined from a selected metal or is configured with a cladding metal jacket over a lead core. The body diameter or "caliber" of the back bullet is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The body of back bullet 640 lacks a conventional pointed nose and terminates at the front (or distally) in a reduced-diameter substantially planar meplat front surface defining a circular surface symmetrically defined around the bullet's central axis, and that meplat front surface 642 is preferably or forced to snugly abut and support the flat base 328 of the coaxially aligned front bullet 320, although an inter-bullet gap (as seen in Fig. 9C) may be defined therebetween or filled with an optional wadding disc (not shown). The distal tapered ogive portion of the body of the back bullet 640 which terminated distally in meplat 642 is in fluid communication with the longitudinal lumen in the cartridge neck 158 and permits expanding gas to flow or pass distally through around back bullet 640 to pressurize the space behind front bullet 320, when fired. The proximal or rearward portion of the body of back bullet 640 preferably has rear cylindrical sidewall segment terminating proximally in a tapered sidewall with a reduced diameter substantially planar transverse rear end to provide a boat-tail back bullet configuration.

[0078] The method of assembling the duplex ammunition 600 as illustrated in Figs 12A-12G is similar to that described above and illustrated in Figs 10D and 10E. Once a standard military cartridge case 150 has been primed (not shown), a selected propellant or powder charge 460 (e.g., Hodgdon H4350 or H 000 smokeless powder) is deposited into the case's interior volume through the cartridge case mouth 154 (see Fig. 12G). Next, proximal "boat tail" base of back bullet 640 is placed over case mouth 154 and back bullet 640 is pressed axially and proximally along axis 150A into the cartridge case mouth so that back bullet 640 is forced into the cartridge case's mouth and the back bullet's distal or forward front surface (see meplat 642 in Fig. 12G) is recessed into the cartridge case mouth 154 and driven 150 thousandths of an inch into the case neck 158, such that approximate^ 50 thousandths of the case neck interior is uncovered by the now inserted back bullet 640, and the back bullet 640 is supported in this orientation partly by the powder charge within the case. Next, a front bullet's proximal flat base 328 is inserted into the cartridge case mouth 154 and front bullet 320 is driven proximally down farther than 150 thousandths (e.g., 280 thousandths) to force base into engagement against the back bullet's meplat 642. Referring now to the photographs in Figs 12E- 12G, Back or rear bullet 642 may be fabricated from a Sierra Match King .308 OTM projectile (normally 1.489 inches long) which has had the open tip cut or milled off to leave a substantially cylindrical bullet body with a slight tapered ogive in the distal sidewall and an overall length of 1.050 inches. That nearly cylindrical bullet body is then inserted distal end first into a hemispherical rounding die to create a distally bulging dome shape from the exposed lead core at the distal end 642B, as shown in Fig. 12E. The distal meplat surface 642 (best seen in Fig. 12G) may be then applied by a die or by forcing the flat base 328 of front bullet 320 against the softer lead core of back bullet to create transverse circular meplat 642, which preferably has a planar diameter of at least 0.150 inches (or about half the diameter or caliber of the body of back bullet 640.

[0079] During assembly of duplex cartridge 600, the front bullet's proximal flat base 328 drives against and is received against the rear bullet's substantially planar "wad-cutter" like meplat front surface 642 when the front bullet 320 is recessed into the cartridge case mouth and driven 280 thousandths of an inch into the case neck 158. Once the duplex cartridge 600 is assembled, the case neck 158 supports the rear of front bullet 320 but the front of rear bullet 640 is driven into the case enough to create an annular gas bypass lumen around the distal end 642 of front bullet 640. As shown in Figs. 12B and 12D (and in the photograph of cut-away casing 150V revealing the cartridge's interior of Fig 12F) a gas bypass annular lumen is provided at the reduced diameter ogive in the distal end 642 of back bullet 640 to vent ignition gas toward the base 328 of front bullet 320, and the transverse gap width of that gas bypass lumen is between five thousandths and ten thousandths of an inch. Thus, the gas bypass lumen area for the exemplary cartridge 600 of Figs 12A-12G is defined by an annulus having an inside diameter of .308 inches and an outside diameter of .313 and .318 inches.

[0080] Figs. 12B and 12D show how the annular gas bypass lumen around the distal end of back bullet 640 configured and located to allow an optimal amount of propellant gas to bypass rear projectile 640 when the propellant is ignited by the primer (not shown). When the primer ignites the propellant charge 460, the back bullet 640 is pushed distally and out of the case mouth 154 but while the back bullet is still within case 150, the annular bypass lumen provides momentary passages for the expanding gas from the burning powder 460 which pressurizes the space behind the front bullet (e.g., 320) and drives it down the barrel (e.g., 14) to create an inter- bullet gap (e.g., "IBG" 480) between the front bullet 320 and the rear bullet 640.

[0081] Gas bypass duplex projectile assembly 600 also provides a

surprisingly effective method for creating separation (e.g., "IBG" 480) between the first bullet 320 and second bullet 640 within the bore (e.g., of rifle 10). Upon firing, the cartridge's ignited powder 460 creates an expanding gas bubble which initially urges both the front and back bullets distally into the barrel's leade (not shown), where front bullet 320 engraves itself on the rifling and begins to accelerate both in its stabilizing rotation about the bullet's central axis and in its travel distally down the bore toward the muzzle. Front bullet 320 is pushed by back bullet 640 and by the expanding gas from the propellant. The annular gas bypass lumen or gas-ducting channel defined around the forward surface 642 of back bullet 640 directs expanding gas to flow distally into the barrel behind the distally moving front bullet 320 to pressurize the base 328 of the front bullet and force it distally down the bore while back bullet 640 is moving slightly more slowly, thereby creating the desired inter-bullet gap (e.g., "IBG" 480) between the distally forced accelerating front bullet 320 and the distal or front edge 642 of the slower back bullet as both bullets travel distally down the bore. This inter bullet gap (e.g., "IBG" 480) defines a captive or trapped volume of expanding gas between the front and back bullets as both travel distally down the bore and allows each bullet to accelerate and engage the barrel's rifling independently. Within the barrel (e.g., 14), each bullet 320, 640 is also independently spin stabilized by the rifling, so the rifling twist rate need not be optimized for a very, very long and heavy (e.g., 350 grains) single bullet (or abutting bullets which spin and act as one) and instead a standard twist rate stabilizes front bullet 320 separately and independently from rear bullet 640.

[0082] Figs 13A - 13D illustrate yet another gas bypass embodiment for the duplex projectile system ammunition assembly 700, where front bullet 320 is preferably configured as a spitzer with a flat circular base and the body of front bullet 320 has a tapered or contoured ogive terminating distally in a distal solid tip or a front facing cavity or opening symmetrically defined around the central axis, and the front facing cavity may be configured to receive a polymer ballistic tip insert (not shown). The diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm) for use in a selected rifle (e.g., 10). The central portion of the body of front bullet 320 may optionally include a sidewall segment carrying a plurality of circumferential grooves of shallow depth and spaced longitudinally along the bullet sidewall from one another or a knurled segment (not shown). The proximal or rearward portion of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a transverse flat, circular base 328 which is symmetrically defined around the front bullet body's central axis, and the substantially planar transverse rear flat base surface 328 is configured to abut a substantially planar ported front surface of the back bullet 740, although an inter-bullet starter gap may be defined therebetween or filled with an optional wadding disc (not shown).

[0083] Back bullet 740 is fabricated or machined from a selected metal or is configured with a cladding metal jacket over a lead core. The diameter or "caliber" of the back bullet is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The body of back bullet 740 lacks a conventional rounded or tapered nose and terminates at the front (or distally) in a full-diameter substantially planar "wad-cutter" front surface symmetrically defined around the bullet's central axis, and that "wad-cutter" front surface is preferably configured to snugly abut and support the flat base of the coaxially aligned front bullet 320, although an inter-bullet gap (as seen in Fig. 9C) may be defined therebetween or filled with an optional wadding disc (not shown). The body of the back bullet 740 preferably includes a sidewall segment carrying a plurality of radially spaced longitudinal grooves or vias 744 which define longitudinal gas bypass ports or lumens in fluid communication with the space behind the front bullet 320. Those longitudinal lumen-defining grooves 744 direct expanding gas to pass distally through around and through back bullet 740 to pressurize the space behind front bullet 320, when fired. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a tapered sidewall with a reduced diameter substantially planar transverse rear end to provide a boat-tail back bullet configuration.

[0084] The method of assembling the duplex ammunition 700 of Figs 13A-D is similar to that described above and illustrated in Figs 10D and 10E. Once a standard military cartridge case 150 has been primed (not shown), a selected propellant or powder charge 460 (e.g., Hodgdon H4350 smokeless powder) is deposited into the case's interior volume through the cartridge case mouth 154. Next, proximal base of back bullet 740 is placed over case mouth 154 and back bullet 540 is pressed axially and proximally along axis 150A into the cartridge case mouth so that back bullet 740 is forced into the cartridge case's mouth and the back bullet's distal or forward substantially planar "wad-cutter" front surface is recessed into the cartridge case mouth 154 and driven 150 thousandths of an inch into the case neck 158, such that approximately150 thousandths of the case neck interior is uncovered by the now inserted back bullet 740. Next, a front bullet's proximal flat base 328 is inserted into the cartridge case mouth 154 and front bullet 320 is driven proximally down and preferably seated into abutment with the back bullet's substantially planar "wad-cutter" front surface so that the front bullet's proximal flat base 328 is received against the rear bullet's substantially planar "wad-cutter" front surface when the front bullet 320 is recessed into the cartridge case mouth and driven 150 thousandths of an inch into the case neck 158. Once the duplex cartridge 700 is assembled, the case neck 158 supports the rear of the front bullet and the front of the rear bullet simultaneously, and (as shown in Fig. 13B) the gas bypass grooves or lumens 744 are provided to vent ignition gas toward the base 328 of front bullet 320.

[0085] Figs. 13B and 13D show how the gas bypass lumens within back bullet 740 are configured and located to allow an optimal amount of propellant gas to bypass rear projectile 740 when the propellant is ignited by the primer (not shown). When the primer ignites the propellant charge 460, the back bullet 740 is pushed distally and out of the case mouth 154 but while the back bullet is still within case 150, the bypass grooves or lumens 744 provide momentary passages which direct or aim the expanding gas from the burning powder 460 to pressurize the space behind the front bullet (e.g., 320) and drives it down the barrel of rifle 10 to create an inter-bullet gap between the front bullet 320 and the rear bullet 740.

[0086] Gas bypass duplex projectile assembly 700 also provides a

surprisingly effective method for creating separation (e.g., "IBG" 480) between the first bullet 320 and second bullet 540 within the bore (e.g., of rifle 10). Upon firing, the cartridge's ignited powder 460 creates an expanding gas bubble which initially urges both the front and back bullets distally into the barrel's leade (not shown), where front bullet 320 engraves itself on the rifling and begins to accelerate both in its stabilizing rotation about the bullet's central axis and in its travel distally down the bore toward the muzzle. Front bullet 320 is pushed by back bullet 740 and by expanding gas from grooves 744. The gas bypass lumens or gas-ducting channels 744 are defined in the distal or forward surface of back bullet 740 to allow expanding gas flowing distally into the barrel behind the distally moving front bullet 320, to pressurize the base 328 of the front bullet and force it distally down the bore while back bullet 740 is moving slightly more slowly, thereby creating an inter-bullet gap between the distally forced accelerating front bullet 320 and the distal or front edge of the slower back bullet 740 as both bullets travel distally down the bore. This inter bullet gap defines a captive or trapped volume of expanding gas between the front and back bullets as both travel distally down the bore and allows each bullet to accelerate and engage the barrel's rifling independently. Within the barrel (e.g., of rifle 10), each bullet 320, 740 is also independently spin stabilized by the rifling, so the rifling twist rate need not be optimized for a very, very long and heavy (e.g., 350 grains) single bullet (or abutting bullets which spin and act as one) and instead a standard twist rate stabilizes front bullet 320 separately and independently from rear bullet 740.

[0087] Finally, Figs 14A and 14B illustrate another alternative embodiment for the duplex projectile system ammunition assembly 800, where front bullet 320 is preferably configured as a spitzer with a flat circular base and the body of front bullet 320 has a tapered or contoured ogive terminating distally in a distal solid tip or a front facing cavity or opening symmetrically defined around the central axis, and the front facing cavity may be configured to receive a polymer ballistic tip insert (not shown). The diameter or "caliber" of the front bullet body is preferably selected from among SAAMI standard calibers (e.g., nominally 0.308 inches or 7.62mm) for use in a selected rifle (e.g., 10). The central portion of the body of front bullet 320 may optionally include a sidewall segment carrying a plurality of circumferential grooves of shallow depth and spaced longitudinally along the bullet sidewall from one another or a knurled segment (not shown). The proximal or rearward portion 326 of the body of the front bullet preferably has rear cylindrical sidewall segment terminating proximally in a transverse flat, circular base 328 which is symmetrically defined around the front bullet body's central axis, and the substantially planar transverse rear flat base surface 328 is configured to abut a substantially planar ported front surface 842 of back bullet 840, although an inter-bullet starter gap may be defined therebetween or filled with an optional wadding disc (not shown).

[0088] Back bullet 840 is fabricated or machined from a selected metal or is configured with a cladding metal jacket over a lead core. The diameter or "caliber" of the back bullet is substantially identical to the front bullet body diameter (e.g., for the illustrative example, nominally 0.308 inches or 7.62mm). The body of back bullet 840 lacks a conventional rounded or pointed nose and tapers slightly to terminate at the front (or distally) in a nearly full-diameter substantially planar "wad- cutter" front surface symmetrically defined around the bullet's central axis, and that "wad-cutter" front surface 842 is preferably configured to snugly abut and support the flat base of the coaxially aligned front bullet 320, although an inter-bullet gap may be defined therebetween or filled with an optional wadding disc (not shown). The body of the back bullet 840 preferably includes a sidewall segment carrying a plurality of radially spaced longitudinal grooves or vias 844 which define longitudinal lumens in fluid communication with the space behind the front bullet 320 and the propellant. Those longitudinal lumen-defining grooves 844 direct expanding gas to pass distally through around and through back bullet 840 to pressurize the space behind front bullet 320, when fired. The proximal or rearward portion of the body of the back bullet preferably has rear cylindrical sidewall segment terminating proximally in a tapered sidewall with a reduced diameter substantially planar transverse rear end to provide a boat-tail back bullet configuration.

[0089] For the gas-bypass embodiments illustrated in Figs 1 1A - 14B, the duplex projectile system ammunition assembly (e.g., 500, 600, 700 or 800) a taper crimp is preferably applied to secure the case neck's retention of front bullet 320. Preferably, a boron nitride coating is applied to at least front bullet 320, preferably with a coating thickness of approximately one micron, which enhances static friction between the sidewall of front bullet 320 and the interior surface of the case neck 158. At present, an alternative preferred propellant charge (for 160, 260, 360 and 460) is about 23 grains of H-1000 powder (which may be seen by those of skill in the art to be a powder selection providing a slower than expected burn rate.)

[0090] The duplex projectile system ammunition assembly (e.g., 500, 600, 700 or 800) of the present invention has been configured to provide a surprising advancement in extreme impact subsonic ammunition, wherein the subsonic embodiment of the ammunition carries front and back projectiles having a combined total weight of 350 grains or more, and when fired, the front and back bullets impact in very close proximity to each other, delivering dramatic results on a target. Each duplex cartridge (e.g., in 308Win or 7.62 NATO) will function and fully stabilize in a .308 Win.- based semi-auto platform rifle (e.g., 10) with no modifications to the rifle gas system, so subsonic duplex loads (e.g., 500, 600, 700 or 800) and supersonic loads may be fired from the same magazine with no other special considerations. For the embodiments illustrated in Figs 1 1A-14B, the duplex projectile system ammunition assembly (e.g., 500, 600, 700 or 800) of the present invention provides projectiles optimized to be compatible with 1-12 and faster twist rate barrels (e.g., 14).

[0091] The front bullet (e.g., 320) is designed to shoot smaller groups and is assigned the designation of the ammunition's "zero." The trailing, second or back bullet (e.g., 540) is constructed as a wadcutter/full-diameter/open-tip. When the shooter or user observes groups on the target, it is easy to distinguish each bullet's impact (as described above and illustrated in Figs 9E-9G). This ammunition is engineered and intended for use at relatively short ranges (e.g., up to 100 yards), but can be used to engage targets at 200 yds. For example, in a hunting

application, the impact distance between the front and the rear bullet is engineered to produce a dual-hit head shot on a pig size target with "double-tap" efficiency from a single fired .308 round. Testing has shown that a full 50-round box of the duplex ammo of the present invention (having 100 projectiles) will produce a composite group of less than 3 inches at 50 yards.

[0092] The kinetic energy delivered to the target when using the ammunition configuration of the present invention is superior. For example, comparing the cartridges described above (e.g., 500, 600, 700 or 800) to "300 Blackout"

ammunition, the BLACKOUT subsonic ammunition fires a 200-grain bullet at 1075 fps to provide 471 ft-lbs of muzzle energy. BLACKOUT supersonic ammunition has a 110-grain bullet at 2300 fps which provides 823 ft-lbs of muzzle energy. The ammunition of the present invention (subsonic) provides: 350 grain total bullet mass (2 projectiles) at 1075 fps = 816 ft-lbs of muzzle energy. So the subsonic 308 load of the present invention provides superior energy on target (816 ft-lbs) compared to the subsonic 300 Blackout load, and substantially equals the supersonic Blackout load's energy at 100 yards.

[0093] In the illustrated embodiments, the duplex projectile system of the present invention (e.g., 100, 200, 300, 500, 600, 700, or 800) is optimized to provide subsonic ammunition which is adapted for use in a standard rifle (e.g., 10) equipped with a suppressor (e.g., 12) or silencer. When a gas operated rifle (e.g. 10) is equipped with a suppressor (e.g., 12). The duplex projectile system of the present invention (e.g., 100, 200, 300, 500, 600, 700, or 800) creates significantly more gas port pressure than standard subsonic ammo, thus allowing standard rifles (e.g., 10) to function without requiring any gas system adjustment or requiring a substitution of louder supersonic ammunition. This benefit is important because the first and second bullets of the duplex system (e.g., 100, 200, 300, 400, 500, 600, 700, or 800) weigh more than any single bullet ever manufactured for use in a standard rifle 10. The firing of the duplex (front and rear) bullets require a higher gas pressure to reach the selected subsonic velocity desired (e.g. 1050 fps). This higher gas pressure makes it possible for an unaltered standard rifle 10 to function without adjusting the gas system. No other subsonic or volley ammunition can provide reliable gas system operation and consistent shot-to-shot subsonic accuracy in standard rifles.

[0094] Significantly, this level of performance (as illustrated in Figs 9E-9G is achieved in part because the barrels in standard rifles provide surprising stability for the first and second projectiles of the duplex projectile system (e.g., 100, 200, 300, 400, 500, 600, 700, or 800 ). A standard rifle (e.g., 0) typically has a rifling twist rate of between one in ten inches to one in twelve inches and is designed to stabilize a single projectile of 147 grains to 175 grains travelling at the standard supersonic velocity (e.g., 2650fps-2800fps). In the present invention, the front and back bullets are stabilized in a standard twist-rate rifle at the selected subsonic velocity (e.g.

1050 fps). The applicants have discovered how to provide the enhanced terminal ballistic benefits of firing a single long and heavy bullet at subsonic velocities, but without requiring a different barrel; a single long bullet that weighed the same amount as the duplex system's front and back bullets (e.g., 350 Gr.) would be too long to be stabilized in a standard twist-rate rifle (e.g., 10). [0095] In the prototypes developed and tested so far, the preferred velocity (e.g. 1050 fps) for the bullets in the duplex projectile system (e.g., 100, 200, 300, 400, 500, 600, 700, or 800 ) was generated with a propellant charge (e.g., 360) comprising 20 grains of H4350 powder when used with a Large Rifle Magnum primer. The front and back bullets of the duplex system (e.g., 100, 200, 300, 400, 500, 600, 700, or 800) can be manufactured precisely and economically by standard methods including casting, swaging or pressing metal alloy components into the desired configurations.

[0096] Having described preferred embodiments of a new and improved method, it is believed that other modifications, variations and changes will be suggested to those skilled in the art in view of the teachings set forth herein. It is therefore to be understood that all such variations, modifications and changes are believed to fall within the scope of the present invention as defined by the following claims.

Claims

WE CLAIM:

1 . A duplex projectile system (e.g., 100, 200, 300, 400, 500, 700) for use in a standard military or tactical rifle system (e.g., 10) of a selected caliber, comprising:

(a) a cartridge case (e.g., 150) with a substantially cylindrical body which is symmetrical about a central axis extending from a substantially closed proximal head to a substantially open distal mouth or lumen, where the body defines an interior volume for containing and protecting a propellant charge, and wherein the cartridge neck is configured to be substantially cylindrical segment having a cylindrical interior lumen in said selected caliber extending from the distal neck end which defines the neck lumen, and wherein the cartridge neck lumen has an interior sidewall with a selected axial neck length; and

(b) a front bullet and a back bullet coaxially aligned with one another and with the case's central axis and held in the case neck (e.g., 158) by inwardly squeezing force applied via the case neck, said case neck squeezing force bearing

simultaneously upon the front bullet (e.g., 120) and the back bullet (e.g., 140).

2. The duplex projectile system of claim 1 , wherein said front bullet (e.g., 120, 220) has a body with a tapered or contoured ogive terminating distally in a tip, and wherein said front bullet body has a proximal or rearward portion with a rear cylindrical sidewall segment terminating proximally in a tapered sidewall segment which then transitions to a rearwardly projecting frustoconical boat tail (e.g., 126, 226) which is symmetrically defined around the front bullet body's central axis, and the frustoconical boat tail terminates proximally or rearwardly in a substantially planar transverse rear end or surface configured to be received snugly within a front cavity of the back bullet (e.g., 140, 240).

3. The duplex projectile system of claim 2, wherein said front bullet (e.g., 120, 220) has an open distal tip which defines a front facing cavity or opening symmetrically defined around the bullet's central axis,

4. The duplex projectile system of claim 2, wherein said back bullet (e.g., 140, 240) is configured with a full caliber sidewall having distal open tip (e.g., 142, 242) and lacking the conventional rounded or tapered nose and terminates at the front (or distally) in a very wide full caliber distal or front-facing concavity which defines a front facing opening symmetrically defined around the bullet's central axis, and that front facing concavity is precisely configured to snugly receive, center and support the rearwardly projecting frustoconical boat tail of the coaxially aligned front bullet.

5. The duplex projectile system of claim 1 , wherein said front bullet (e.g., 320) has a body with a tapered or contoured ogive terminating distally in a full metal jacket clad tip or an open tip which defines a front facing cavity or opening where the front bullet's body is symmetrically defined around the bullet's central axis, and wherein said front bullet body has a proximal or rearward portion with a rear cylindrical sidewall segment terminating proximally in a substantially planar flat transverse base (e.g., 328) which is symmetrically defined around the front bullet body's central axis, and the front bullet's flat base provides a surface configured to be received snugly against a front substantially planar wadcutter surface (e.g., 342, 442, 542, 742) of the back bullet (e.g., 340, 440, 540, 740).

6. The duplex projectile system of claim 4, wherein said back bullet (e.g., 340) is configured with full caliber sidewall extending from the distal or front substantially planar wadcutter surface and symmetrically defined around the back bullet's central axis, wherein the front facing substantially planar full caliber wadcutter surface of the back bullet is precisely configured to snugly receive and support the proximal flat base of the coaxially aligned front bullet.

7. The duplex projectile system of claim 1 , wherein said front bullet is fabricated from lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass.

8. The duplex projectile system of claim 1 , wherein said back bullet is fabricated from lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass.

9. The duplex projectile system of claim 1 , wherein cartridge case is manufactured to one of (a) 7.62 NATO ammunition specifications, (b) SS109/M855 5.56 NATO ammunition specifications or A191/MK 248 .300 Winchester magnum ammunition specifications.

10. A method for making a duplex projectile system for use in a standard military or tactical rifle system in a selected caliber, comprising the method steps of:

(a) providing a cartridge case with a substantially cylindrical body which is symmetrical about a central axis extending from a substantially closed proximal head to a substantially open distal mouth or lumen, where the body defines an interior volume for containing and protecting a propellant charge, and wherein the cartridge neck is configured to be substantially cylindrical segment extending from the distal neck end which defines the neck lumen rearwardly or proximally to an angled shoulder segment which flares out to the cylindrical body sidewall, and wherein the cartridge neck has a neck lumen interior sidewall with a selected axial neck length;

(b) providing a front bullet with a rear or proximal surface of the selected caliber;

(c) providing a back bullet with substantially cylindrical full caliber sidewall terminating in a distal front surface on one end and a proximal base on the other end;

(d) inserting a selected quantity of a selected propellant or powder into the case's interior volume through the cartridge case mouth; and

(e) inserting the back bullet's proximal base into the cartridge case mouth;

(f) driving the back bullet axially and down into the cartridge case's mouth so that the back bullet's distal or forward front surface is recessed into the cartridge case mouth and driven approximately half way along the case neck sidewall's axial length so that about half the case neck sidewall remains uncovered by the now inserted back bullet, when looking into the cartridge case mouth.

11. The method for making a duplex projectile system of claim 10, further comprising the steps of:

(g) inserting the front bullet's rear or proximal end into the cartridge case mouth;

(h) driving the front bullet axially and down into the cartridge case neck far enough to engage or provide a selected gap from the back bullet's distal front surface so that the case neck supports the rear of the front bullet and the front of the rear bullet and coaxially aligns the front bullet and the rear bullet with the case body's central axis, thereby causing a substantially uniform case neck tension to hold the front bullet and back bullet in the case neck by inwardly squeezing force applied via the case neck simultaneously bearing upon the front bullet and the back bullet.

12. The method for making a duplex projectile system of claim 1 1 , wherein said back bullet is driven into said case to expose approximately 0.150 inches of case sidewall.

13. The method for making a duplex projectile system of claim 12, wherein said front bullet is driven approximately 0.150 inches into said case mouth to abut said back bullet's distal front surface, while not driving said back bullet further into said cartridge case's interior.

14. The method for making a duplex projectile system of claim 10,

(h) driving the front bullet axially and down into the cartridge case neck far enough to engage the back bullet's proximal front meplat surface (e.g., 642) so that the case neck supports the front bullet and drives the distal meplat of the back bulled proximally and out of contact with the neck, thereby defining an annular gas- bypass lumen between the cartridge and the back bullet's distal meplat.

15. The method for making a duplex projectile system of claim 14, wherein said annular gas-bypass lumen between the cartridge and the back bullet's distal meplat has an outside diameter of 0.313 to 0.3 8 inches for a back bullet caliber of 0.308 inches.

16. A duplex projectile system for use in a standard military or tactical rifle system of a selected caliber, comprising:

(a) a cartridge case with a substantially cylindrical body which is symmetrical about a central axis extending from a substantially closed proximal head to a substantially open distal mouth or lumen, where the body defines an interior volume for containing and protecting a propellant charge, and wherein the cartridge neck is configured to be substantially cylindrical segment having a cylindrical interior lumen in said selected caliber extending from the distal neck end which defines the neck lumen rearwardly or proximally to an angled shoulder segment which flares out to the cylindrical body sidewall, and wherein the cartridge neck has a neck lumen interior sidewall with a selected axial neck length;

(b) a front bullet and a back bullet coaxially aligned with one another and with the case's central axis; and

(c) wherein the duplex projectile system is optimized to provide subsonic ammunition which is adapted for use in a standard rifle equipped with a suppressor or silencer.

17. The duplex projectile system of claim 16, wherein said front and back bullets generate a higher gas pressure and when fired and reach a selected subsonic velocity (e.g. 1050 fps) while functioning in a standard rifle without requiring any adjustment to rifle's gas system.

18. The duplex projectile system of claim 16, wherein said front and back bullets are stabilized to provide consistent shot-to-shot subsonic accuracy in a standard rifle's barrel.

19. The duplex projectile system of claim 18, wherein said front and back bullets, are aligned and configured within said casing to define at least one gas bypass lumen so that, when fired, propellant gasses bypass the back bullet and pressurize the rear surface or base (e.g., 328) of the front bullet (e.g., 320) which is thereby forced into the rifle's barrel (e.g., 14) by said bypassing gas;

and wherein said back bullet is then driven into said barrel thereby trapping said bypassing gas between said front bullet and said back bullet, to provide an inter bullet gap (e.g., "IBG" 480) between said front bullet and said rear bullet;

whereby said front bullet and said rear bullet each engage the barrel's rifling separately and independently, and each are individually stabilized by the rifling.

20. The duplex projectile system of claim 16, wherein said back bullet includes at least one longitudinal gas port defined between the back bullet's sidewall and the back bullet's distal end;

said gas port being configured to allow expanding gas to pass distally beside or through the back bullet's body and pressurize the space behind the front bullet and drive the front bullet distally down the firearm's bore;

wherein said port is configured to direct said gas to force an inter-bullet gap (e.g., "IBG" 480) between the front bullet and the back bullet upon firing; and

(d) wherein the duplex projectile cartridge is optimized to provide subsonic ammunition which is adapted for use in a standard rifle equipped with a suppressor or silencer.

21. The duplex projectile system of claim 20, wherein said front and back bullets generate a higher gas pressure and when fired and reach a selected subsonic velocity (e.g. 1050 fps) while functioning in a standard rifle without adjusting the gas system.

22. The duplex projectile system of claim 21 , wherein said front and back bullets are stabilized to provide consistent shot-to-shot subsonic accuracy in standard rifles.

23. The duplex projectile system of claim 22, wherein said front bullet is fabricated from lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass.

24. The duplex projectile system of claim 22, wherein said back bullet is fabricated from lead, tungsten, copper alloy cladded lead, copper alloy cladded tungsten or C36000 brass.

25. The duplex projectile system of claim 16, wherein said cartridge case is manufactured to one of (a) 7.62 NATO ammunition specifications, (b) SS109/M855 5.56 NATO ammunition specifications or A191/MK 248 .300 Winchester magnum ammunition specifications.