US3771415A

US3771415A - Rifle conversion assembly

Info

Publication number: US3771415A
Application number: US00224208A
Authority: US
Inventors: H Into; R Costello
Original assignee: Colt Industries Operating Corp
Current assignee: CFPI Inc A CORP OF; COLT LICENSING A DE Ltd PARTNERSHIP LP; CII Holdings Inc
Priority date: 1972-02-07
Filing date: 1972-02-07
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13

Abstract

An assembly is provided for converting a firearm of standard caliber to one of a smaller caliber and includes a conversion chamber adaptor with a bolt mounted for movement between recoil and battery positions. A backplate damper assists in maintaining the operating components in assembly and absorbs recoil impact energy of the recoiling mass upon firing of the weapon. When the conversion assembly is installed in a weapon having a buffer disposed rearwardly of its chamber, the backplate damper additionally serves to transmit that energy to the buffer.

Description

United States Patent n91 Into et al.

[ Nov. 13, 1973 1 1 RIFLE CONVERSION ASSEMBLY [75] Inventors: Henry A. Into; Richard L. Costello,

both of Hartford, Conn. 06102 [73] Assignee: Colt Industries Operating Corp.,

Hartford, Conn.

{22] Filed: Feb. 7, 1972 [21] Appl. No.: 224,208

[52] US. Cl 89/16, 42/25, 42/49 A, 89/29, 89/128, 89/197, 89/198 [51] Int. Cl. F410: 21/12 [58] Field of Search 42/77; 89/14 R, 16, 89/29, 128,194, 197, 198, 199

[56] References Cited UNlTED sTATEs PATENTS 2,290.156 7/1942 Brewer 89/194 FOREIGN PATENTS OR APPLICATIONS 1,428,611 11/1968 Germany 89/29 e, r, i 1.4 1L, 5W9; e f" 1/ H? 78 m Av 3 w Z17 /f O 6 O 1,093,265 11/1960 Germany ..42/77 Primary Examiner-Stephen C. Bentley Attorney.lohn M. Prutzman et al.

[57] ABSTRACT 8 Claims, 5 Drawing Figures PAIENIEBNUHSISH 3.771.415

SHEEI 1 [1F 2 RIFLE CONVERSION ASSEMBLY This invention generally relates to gas operated firearms and particularly to a conversion assembly for converting a firearm to fire smaller size ammunition.

A primary object of this invention is to provide a new and improved conversion assembly of a compact, rugged construction which may be quickly and easily manufactured at relatively low cost and which is suited for converting standard weapons such as rifles to fire a relatively inexpensive bullet of smaller caliber for training purposes and the like.

Another object of this invention is to provide a new and improved conversion assembly featuring a combination backplate and damper device which not only assists in maintaining the conversion components in assembly but also absorbs and transmits bolt recoil impact energy to a buffer of the firearm in which the assembly is installed.

A further object of this invention is to provide a conversion assembly of the type described having a new and improved cartridge ejector and extractor arrangement.

Another object of this invention is to provide such a conversion assembly having a new and improved firing pin arrangement particularly suited to minimize any tendency of hammer blows to weaken or fracture thin portions of the firing pin shank. Included in this object is the aim of permitting a relatively large fillet radius between the shank and an enlarged head of the firing pin to further minimize undesired failures and at the same time to eliminate any spreading of its return spring coils and binding of the pin.

Other objects will be in part obvious and in part pointed out in more detail hereinafter.

A better understanding of the objects, advantages, features, properties and relationships of this invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and is indicative of the way in which the principle of this invention is employed.

In the drawings:

FIG. I is a fragmentary side elevational view, partly broken away and partly in section, ofa rifle with a con version assembly incorporating this invention shown installed in the rifle and illustrated in a battery positipn;

FIG. 2 is a side view, partly broken away and partly in section, showing certain details of a bolt and modified magazine of the conversion assembly of FIG. 1 illustrated in a recoil position;

FIG. 3 is an enlarged view, partly in section, showing a front face of the bolt;

FIG. 4 is an enlarged view, partly broken away and partly in section, of the bolt taken generally along lines 4-4 of FIG. 3 and shown in an intermediate recoiling position relative to a conversion chamber adaptor; and

FIG. 5 is an enlarged view, partly in section, showing a rear face of the bolt.

Referring now to the drawings in detail, a preferred embodiment of this invention is shown incorporated in a conversion assembly generally designated 10. It will be understood that while the conversion assembly is shown installed in a gas operated automatic rifle 12 for purposes of illustration, the conversion assembly of this invention is readily adapted for use in other firearms of a magazine fed, gas operated type which fire from closed bolt position.

To convert the illustrated rifle 12 to fire ammunition of a smaller caliber in the rifle 12 which is chambered for a larger, more expensive caliber bullet, the conversion assembly 10 is installed after a standard bolt carrier group (not shown) of the rifle 12 has been removed. This is accomplished by simply removing the standard rifle magazine from lower receiver 14, pushing out a takedown pin 16 interconnecting the lower receiver 14 to upper receiver [8 and swinging the lower receiver 14 downwardly about pivot pin 20. The standard bolt carrier group may then be withdrawn rearwardly from the upper receiver 18 of the opened weapon and the conversion assembly 10 may be installed.

To facilitate such installation and to ensure ease in removing and otherwise handling the conversion as' sembly 10 without requiring any loose parts, detents, thumb screws, etc. normally associated with devices of this type, the replacement components to be substituted for the standard bolt carrier group to effect the desired conversion are designed as an integral onepiece unit.

This unit includes a conversion chamber adaptor 22 having suitable lugs 24 engageable with conventional bolt locking lugs 26 on a breech end of barrel 28 upon inserting and rotating the adaptor 22 within rifle chamber 30 to lock the adaptor 22 in operative position. (An upper adaptor lug preferably engages an interior re ceiver wall 18a for guiding the assembly I0 during its installation and removal.) Upon adaptor 22 being moved home into the rifle chamber 30, e.g., the assembly 10 is rotated and an upper lug is angularly moved into a generally vertical position with the adaptor lugs 24 lockingly engaged with the bolt locking lugs 26 of the rifle chamber 30. The adaptor 22 has a chamber 32 extending axially forwardly of its rear face 34, and the chamber 32 is bored for a caliber, such as a 0.22 long rifle, e.g., which is smaller than the standard caliber of rifle 12.

A bolt 36 is supported for reciprocating movement between a battery position (shown in FIG. 1 with the bolt 36 engaging rear face 34 of adaptor 22) and a rear recoil position (FIG. 2) and is supported for such movement on a pair of

guide rods

38, 40 received in exposed longitudinally extending grooves 42, 44 of generally "U" shaped cross-section formed in generally diametrically opposed sides of the bolt 36, preferably in a plane disposed at an angle to the horizontal as best seen in FIGS. 3 and 5. The

guide rods

38, 40 are suitably fixed to a rear portion of adaptor 22, as by the illustrated threaded connection, and extend axially rearwardly from the adaptor 22 to a position adjacent a buffer 46 of the rifle 12. The buffer 46 is reciprocably mounted in a tubular body of the lower receiver 14 and is biased forwardly by a spring 48 into a normal position illustrated in FIG. I. In the specifically illustrated embodiment,

guide rods

38, 40 terminate in radially enlarged heads as seen at 50 in FIG. 1 which is disposed in forwardly spaced adjacent relation to the buffer 46 within the confines ofa backplate damper 52. As fully described below, backplate damper 52 is seated on a front face 54 of the buffer 46, and backplate damper 52 serves to engage the recoiling bolt 36 for establishing its recoil position (FIG. 2). A coil compression action spring 56 is provided guidance and support on the guide rod 38 and normally maintains the backplate damper 52 bottomed against the buffer face 54 while urging the bolt 36 into its battery position in engagement with the rear face 34 of the conversion chamber adaptor 22.

A longstanding problem encountered in firearm bolts such as the one illustrated in this embodiment concerns the desirability to provide a relatively large fillet radius at the juncture of the shank 58 of the firing pin 60 (FIG. 2) and its enlarged rear flange or head 62 to minimize any sharp corners which may otherwise result in the shank 58 being broken off from the flange 62 under repeated hammer impact which imposes certain bending moments on the reduced cross-sectional portion of the pin 60. Nonetheless, when the juncture between the shank 58 and the flange 62 is formed with a large fillet radius, end coils of the return spring 64 adjacent the juncture tend to spread and bind up the pin 60 particularly between the pin 60 and rear maximum diameter portion 66 of the firing pin passageway 68. It will be understood that the forward limit of firing pin movement is established by engagement of its rear flange 62 with shoulder 69 in the passageway 68.

To overcome these problems, the illustrated embodiment shows a three-piece firing pin arrangement in FIG. 2 wherein the firing pin 60 is mounted directly in front of the striker 70 which is received in maximum diameter portion 66 of the passageway 68 for limited movement between forward and rear operative limit positions. The forward and rear operative limit positions ofstriker 70 are determined by engagement ofthe striker 70 with the firing pin 60 in its forward limit position and engagement of striker shoulder 74 with a fixed retaining pin 76 between

shoulders

72, 74 ofstriker 70. Upon impact of hammer 78, after a full drop between opposed vertical walls 80, 82 (FIG. of rearwardly extending

longitudinal shoulders

84, 86 of the bolt 36, the pin 60 is driven forwardly from its illustrated rear position to its forward position wherein the tip of the pin 60 protrudes outwardly ofa forward reduced portion 68A of passageway 68 to detonate a primer of a chambered cartridge and fire its bullet.

A relatively large fillet radius is desirably provided at the juncture of firing pin shank 58 and flange 62, without the undesired tendencies of the end coils of return spring 64 spreading within the passageway 68, by virtue of a sleeve 90 fitted in concentric relation about a rear portion of shank $8. Sleeve 90 presents an annular forwardly facing wall serving as a seat for an end of return spring 64 which is thereby positively confined within intermediate passageway portion 688. The opposite end of return spring 64 fits against a bevelled shoulder 92 within the bolt 36 at a juncture between the intermediate passageway portion 688 and the forward reduced portion 68A of passageway 68.

Upon firing, the reaction force of gases formed upon explosion of the chambered round drive bolt 36 rearwardly. Automatic extraction and ejection of empty cartridge cases such as at 94 in FIG. 4, through an ejection port, not shown, is effected by a simplified but significantly compact, rugged arrangement during rearward recoil of bolt 36. An extractor member or claw 96 is pivotally mounted in bolt 36 and urged inwardly toward its major longitudinal axis X-X by an extractor spring 98 biasing a plunger 100 forwardly against a heel of the claw 96, whereby during colt recoil, claw 96 rides down a ramp surface 102 within the conversion chamber adaptor 22 and engages a rim 104 of the spent cartridge case 94 to withdraw it from the adaptor chamber 32.

During continued bolt recoil, the case 94 emerges from adaptor chamber 32 and the base of the case 94 engages an ejection member or lug 106 of magazine 108, whereupon an ejector assist member of paw 110 throws the empty case 94 out the ejection port under the force of its plunger spring 112 which is biased as best seen in FIG. 4 in a manner similar to that shown in connection with the claw 96. The pawl 110 protrudes forwardly of bolt face 114 and provides a cartridge wall engaging portion 116 which is positioned a discrete distance beyond the cartridge rim engaging portion 118 of the claw 96 such that, during recoil, the spring loaded pawl 110 rides down ramp surface 120 in the adaptor 22, engages the wall of the spent case 94 and throws it out at a selected angle through the ejection port when the cartridge base engages the cooperating magazine ejection lug 106.

In this respect, the ejection lug 106 is located adjacent the path movement of the pawl 110, and the pawl 110 is mounted in a predetermined position relative to the extractor claw 96 and the ejection port to ensure a proper angle of ejection. In the specific illustrated embodiment, the pawl 110 is supported for pivotal movement in a generally horizontal plane extending axially of the bolt 36 with the path of pivotal movement of the extractor claw 96 lying in an axial plane extending at an angle of about 22 6 to the horizontal to eject cartridge cases over the extractor claw 96 and out the ejection port with an upward thrust in the embodiment illustrated.

As the bolt 36 is driven back toward its recoil position, the compression action spring 56 coiled about the guide rod 38 is loaded, as spring 56 absorbs part of the recoil energy, and the hammer 78 is returned to its illustrated cocked position upon being engaged by an underlying shoulder 122 on the recoiling bolt 36. When the bolt face 114 clears the magazine 108, which is suitably modified to be installed within the magazine well 124 oflower receiver 14 for storing and feeding rounds ofa desired smaller caliber, a magazine follower spring, not shown, feeds a live cartridge into ready position (as seen in broken lines at 126 in FIG. 2) to be picked up by the bolt 36 during its return forward movement and to be chambered in the adaptor 22 for firing.

To minimize rebound and repeatedly absorb high impact loads responsive to firing and to also effect a smooth counter recoil movement to the bolt 36 under the bias of the compressed action spring 56 with minimal bolt bounce at battery impact, the backplate damper 52 is mounted on the

guide rods

38, 40 at the rear of the conversion assembly 10. This damper not only absorbs recoil impact energy when the recoiling mass strikes the damper 52, but additionally transmits that energy to the rifle buffer 46 and effectively minimizes impact energy return to bolt 36. Accordingly, in accordance with this invention, the damper 52 is seated against the buffer 46 and is supported for limited movement on the

guide rods

38, 40 with the damper 52 nor mally urged forwardly a predetermined distance from its rear limit position by the buffer 46 when bolt 36 is in battery. As best seen in FIG. 1, the extent of damper movement is limited by a clearance gap between abutment walls of rod receiving wells such as at 128 and the heads 50 of the

guide rods

38, 40 which establish a rear limit position for the damper 52. It has been found that a clearance of, say, 0.080 to 0.l00 inch provides satisfactory damping of the recoiling mass and subsequent transmission of its energy to the rifle buffer 46 to bring the bolt 36 to a substantially dead stop for a smooth return motion under the bias of the loaded action spring 56. As the bolt 36 moves forwardly, its face 114 picks up a cartridge such as at 126 from the magazine 108 and chambers the round. The ejection lug 106 on magazine 108 will be seen to be received within a longitudinally extending groove 130 on the bottom surface of the bolt 36 (FIG. 3). As the bolt 36 moved holne into battery position, the claw 96 and ejector assist pawl 110 are biased radially outwardly upon their

engaging ramp surfaces

102 and 120 in adaptor 22 and the con version assembly is conditioned for firing the next round.

In addition to serving as both a backplate and a damper, member 52 additionally has a lug receiving locating slot 132 which cooperates with a buffer catch lug 134 in lower receiver 14 to maintain conversion assembly 10 in operative position. A similar cooperating slot and lug arrangement is provided at the forward end of the assembly (FIG. 2) wherein a bottom surface of adaptor 22 has a longitudinally extending slot 136 which receives an upstanding lug 138 at the front of the top wall of magazine 108 to additionally restrain the installed conversion assembly 10 against undesired angular movement relative to receiver 14. Moreover, hammer movements additionally are of assistance should any minor misalignment of bolt 36 relative to its

guide rods

38, 40 occur, for the

longitudinally extending shoulders

84, 86 on the rear of bolt 36 would then be engaged and cammed back into proper position by hammer 78 during its movement between cocked and firing positionsv A conversion assembly 10 of the type disclosed will be seen to be quickly installed and easily removed from a firearm with which it is being used. To remove the conversion assembly 10, the rifle 12 shown in the specifically illustrated embodiment need only be opened, after the magazine I08 has been removed, and the conversion assembly 10 is then rotated as a unit to disengage its adaptor lugs 24 from the breech end of barrel 28 whereupon the entire conversion assembly 10 is detachable as an integral unit. Moreover, the conversion assembly 10 of this invention may be economically manufactured and, due to its simplified but rugged construction, is particularly suited for extended use with minimum service requirements.

As will be apparent to persons skilled in the art, various modifications, adaptations and variations of the foregoing specific disclosure can be made without departing from the teachings of the present invention.

We claim:

I. A firearm conversion assembly detachably insertable into a chamber ofa firearm receiver upon removal of its standard bolt means for converting the firearm to fire ammunition of a caliber smaller than its standard caliber and comprising a conversion chamber adaptor insertable into the firearm chamber in fixed relation thereto, a bolt, a backplate damper, detachable mounting means releasably mounting the adaptor, bolt and backplate damper in a unitary assembly and including a guide rod detachably connecting the bolt and the backplate damper in assembled relation and supporting the bolt and the backplate damper on the guide rod, the bolt and the backplate damper being reciprocable on the guide rod with the bolt movable between a forward battery position in engagement with the conversion adaptor and a rear recoil position, the guide rod establishing a rear limit position for the backplate damper and cooperating therewith in establishing the rear recoil position of the bolt, and an action spring supported on the guide rod with opposite ends of the spring seated on the bolt and the backplate damper, the spring urging the bolt forwardly toward battery position and urging the backplate damper toward its rear limit position, the backplate damper being engageable with the bolt and serving to absorb recoil impact energy of the recoiling mass when the bolt moves from battery to recoil positions.

2. The assembly ofclaim 1 in combination with a firearm having a buffer engageable with the backplate damper upon installation of the conversion assembly into the firearm chamber, wherein the backplate damper is urged a predetermined distance forwardly of its rear limit positions by the firearm buffer when the bolt is in battery position for minimizing rebound of the bolt during its movements between battery and recoil positions.

3. The assembly of claim 1 wherein the backplate damper includes a lug receiving locating slot for locating and retaining the conversion assembly in operative position relative to the firearm receiver.

4. The assembly of claim 3 further including a modified conversion magazine having an upstanding lug, the conversion adaptor having a bottom surface with a lug receiving locating slot which cooperates with the magazine lug, upon mounting the magazine in the firearm receiver, for locating and retaining the conversion ssembly in operative position relative to the firearm receiver.

5. The assembly of claim 1 wherein the bolt is an elongated member having an axially extending firing pin passageway, a firing pin is received in the passageway for reciprocating movements between forward and rear positions, a hammer actuated striker is mounted in the bolt for movement between forward and rear operative limit positions, a return coil spring is concentrically received in the passageway about the firing pin for urging it toward its rear position and biasing the striker toward its rear limit position for hammer engagement, and a sleeve is received in the passageway surrounding the firing pin and having an annular front wall serving as a return spring seat.

6. The assembly of claim 1 wherein the bolt is an elongated member carrying a cartridge rim-engaging extractor and cartridge wall-engaging ejector assist member, the extractor and the ejector assist member each being supported on the bolt for pivotal movement toward and away from the major longitudinal axis of the bolt, spring means in the bolt normally biasing the extractor and the ejector assist member toward the central longitudinal axis of the bolt, the conversion adaptor having a rear face with a cartridge chamber extending axially forwardly therefrom, and first and second ramp surfaces formed in the adaptor and diverging from the rear face of the conversion adaptor radially outwardly of its chamber for camming the extractor and ejector assist member radially outwardly when the action spring moves the bolt into battery in engagement with the rear face of the conversion adaptor.

7. A firearm conversion assembly detachably insertable into a chamber ofa firearm receiver upon removal of its standard bolt means for converting the firearm to fire ammunition of a caliber smaller than its standard caliber and comprising a conversion chamber adaptor insertable into the firearm chamber in fixed relation thereto, a bolt, a backplate damper, mounting means including a pair of guide rods threadably fixed to the adaptor and supporting the bolt and the backplate damper for reciprocable movement, the bolt being reciprocable between a forward battery position in engagement with the conversion adaptor and a rear recoil position, at least one of the guide rods establishing a rear limit position for the backplate dampler and cooperating with the backplate damper in establishing the rear recoil position of the bolt, an action spring mounted on one of the guide rods between the bolt and the backplate damper urging the bolt forwardly toward battery position and urging the backplate damper toward its rear limit position, the backplate damper being engageable with the bolt and serving to absorb recoil impact energy of the recoiling mass when the bolt moves from battery to recoil positions, the guide rods detachably securing the conversion assembly components in an integral unit for quick and easy installation and removal relative to the firearm receiver.

8. A firearm conversion assembly detachably insertable into a chamber of a firearm receiver upon removal of its standard bolt means for converting the firearm to fire ammunition of a caliber smaller than its standard caliber and comprising a conversion chamber adaptor insertable into the firearm chamber in fixed relation thereto. an elongated bolt including an axially extending firing pin passageway having concentric passageway portions with a rear maximum diameter portion, a

forward reduced diameter portion and an interconnecting portion therebetween of intermediate diameter size, a firing pin in the passageway reciprocable between forward and rear positions, the firing pin having a radially enlarged end flange received in the maximum diameter passageway portion and a shank extending into the forward passageway portion and merging with said end flange with a smoothly contoured fillet radius, a hammer actuated striker received in the rear maximum diameter passageway portion of the bolt for movement between forward and rear operative limit positions, a return coil spring concentrically received in the passageway about the firing pin for urging it toward its rear position and biasing the striker toward its rear limit position for hammer engagement, a sleeve received in the passageway surrounding the firing pin shank and having an annular front wall serving as a return spring seat, the sleeve confining the return spring within the intermediate passageway portion between the front wall of the sleeve and an abutment formed at the juncture between the intermediate and forward passageway portions, mounting means mounting the bolt for reciprocable movement between a forward battery position in engagement with the conversion adaptor and a rear recoil position, a backplate damper supported by the mounting means in a position disposed rearwardly of the bolt, and an action spring between the bolt and the backplate damper urging the bolt forwardly toward battery position, the backplate damper being cngageable with the bolt and serving to absorb recoil impact energy of the recoiling mass when the bolt moves from battery to recoil positions.

4! i i l

Claims

1. A firearm conversion assembly detachably insertable into a chamber of a firearm receiver upon removal of its standard bolt means for converting the firearm to fire ammunition of a caliber smaller than its standard caliber and comprising a conversion chamber adaptor insertable into the firearm chamber in fixed relation thereto, a bolt, a backplate damper, detachable mounting means releasably mounting the adaptor, bolt and backplate damper in a unitary assembly and including a guide rod detachably connecting the bolt and the backplate damper in assembled relation and supporting the bolt and the backplate damper on the guide rod, the bolt and the backplate damper being reciprocable on the guide rod with the bolt movable between a forward battery position in engagement with the conversion adaptor and a rear recoil position, the guide rod establishing a rear limit position for the backplate damper and cooperating therewith in establishing the rear recoil position of the bolt, and an action spring supported on the guide rod with opposite ends of the spring seated on the bolt and the backplate damper, the spring urging the bolt forwardly toward battery position and urging the backplate damper toward its rear limit position, the backplate damper being engageable with the bolt and serving to absorb recoil impact energy of the recoiling mass when the bolt moves from battery to recoil positions.

2. The assembly of claim 1 in combination with a firearm having a buffer engageable with the backplate damper upon installation of the conversion assembly into the firearm chamber, wherein the backplate damper is urged a predetermined distance forwardly of its rear limit positions by the firearm buffer when the bolt is in battery position for minimizing rebound of the bolt during its movements between battery and recoil positions.

7. A firearm conversion assembly detachably insertable into a chamber of a firearm receiver upon removal of its standard bolt means for converting the firearm to fire ammunition of a caliber smaller than its standard caliber and comprising a conversion chamber adaptor insertable into the firearm chamber in fixed relation thereto, a bolt, a backplate damper, mounting means including a pair of guide rods threadably fixed to the adaptor and supporting the bolt and the backplate damper for reciprocable movement, the bolt being reciprocable between a forward battery position in engagement with the conversion adaptor and a rear recoil position, at least one of the guide rods establishing a rear limit position for the backplate dampler and cooperating with the backplate damper in establishing the rear recoil position of the bolt, an action spring mounted on one of the guide rods between the bolt and the backplate damper urging the bolt forwardly toward battery position and urging the backplate damper toward its rear limit position, the backplate damper being engageable with the bolt and serving to absorb recoil impact energy of the recoiling mass when the bolt moves from battery to recoil positions, the guide rods detachably securing the conversion assembly components in an integral unit for quick and easy installation and removal relative to the firearm receiver.

8. A firearm conversion assembly detachably insertable into a chamber of a firearm receiver upon removal of its standard bolt means for converting the firearm to fire ammunition of a caliber smaller than its standard caliber and comprising a conversion chamber adaptor insertable into the firearm chamber in fixed relation thereto, an elongated bolt including an axially extending firing pin passageway having concentric passageway portions with a rear maximum diameter portion, a forward reduced diameter portion and an interconnecting portion therebetween of intermediate diameter size, a firing pin in the passageway reciprocable between forward and rear positions, the firing pin having a radially enlarged end flange received in the maximum diameter passageway portion and a shank extending into the forward passageway portion and merging with said end flange with a smoothly contoured fillet radius, a hammer actuated striker received in the rear maximum diameter passageway portion of the bolt for movement between forward and rear operative limit positions, a return coil spring concentrically received in the passageway about the firing pin for urging it toward its rear position and biasing the striker toward its rear limit position for hammer engagement, a sleeve received in the passageway surrounding the firing pin shank and having an annular front wall serving as a return spring seat, the sleeve confining the return spring within the intermediate passageway portion between the front wall of the sleeve and an abutment formed at the juncture between the intermediate and forward passageway portions, mounting means mounting the bolt for reciprocable movement between a forward battery position in engagement with the conversion adaptor and a rear recoil position, a backplate damper supported by the mounting means in a position disposed rearwardly of the bolt, and an action spring between the bolt and the backplate damper urging the bolt forwardly toward battery position, the backplate damper being engageable with the bolt and serving to absorb recoil impact energy of the recoiling mass when the bolt moves from battery to recoil positions.