US3894489A

US3894489A - Explosive assemblies and method of utilizing the same

Info

Publication number: US3894489A
Application number: US428017A
Authority: US
Inventors: Hans-Joachim Riedl; Heinz Schluter; Walter Trinks; Gerhard Hensel
Original assignee: Nitrochemie Aschau GmbH
Current assignee: Nitrochemie Aschau GmbH
Priority date: 1961-05-18
Filing date: 1965-01-21
Publication date: 1975-07-15
Anticipated expiration: 1992-07-15

Abstract

1. In an explosive assembly, in combination, primary and secondary explosive charges; directing means common to both of said charges and cooperating therewith for directing the blasts from both of said charges along the same path in the same direction toward a given target; and means operatively connected to both charges for igniting said secondary explosive charge at an interval of 10-150 microseconds subsequent to the ignition of said primary charge, whereby the blast from said secondary charge will enter an area which has previously been entered by the blast of said primary charge to augment the destructive effects of said primary charge.

Description

Riedl et al.

i 1 EXPLOSIVE ASSEMBLIES AND METHOD OF UTILIZING THE SAME [75] Inventors: Hans-Joachim Riedl,

Recklinghausen; Heinz Schliiter, Merf'eld-Dulmen; Walter Trinks, Rhondorf, all of Germany; Gerhard Hensel, deceased, late of Geesthacht, Elbe, Germany, by Margarete Hensel, administratrix [73 Assignee: Nitrochemie GmbH, Aschau, near Krainberg am Inn, Germany [22] Filed: Jan. 21,1965

[2]] Appl. No.: 428,017

Related U.S. Application Data [63] Continuation-in-part of Ser. No. |ll,ll2, May 18,

1961 abandoned.

[52] U.S. Cl 102/24 HC [51] Int. Cl. F42b 3/08 [58] Field of Search 102/24 HC, 56

[56] References Cited UNITED STATES PATENTS 1804823 9/1957 Jablansky 102/49 S July 15, 1975 FOREIGN PATENTS OR APPLICATIONS 341,743 l0/l959 Switzerland 102/24 HC Primary E \'uminer-Verlin R. Pendegrass Attorney, Agenl, or Firm-Michael S. Striker EXEMPLARY CLAIM 1. In an explosive assembly, in combination, primary and secondary explosive charges; directing means common to both of said charges and cooperating therewith for directing the blasts from both of said charges along the same path in the same direction toward a given target; and means operatively connected to both charges for igniting said secondary explosive charge at an interval of 10-150 microseconds subsequent to the ignition of said primary charge, whereby the blast from said secondary charge will enter an area which has previously been entered by the blast of said primary charge to augment the destructive effects of said primary charge.

5 Claims, 4 Drawing Figures PI'UENTEU 3, 894,489

SHEET 1 Fig! Fig.2

1/ 4 v 4 h I!! 5 "Hm "1 "1' I ELM] m. 'h hlilh.

EXPLOSIVE ASSEMBLIES AND METHOD OF UTILIZING THE SAME This application is a continuation-in-part of application Ser. No. 111,112 filed May 18, 1961 and entitled Method Of Directing And Increasing The Effects Of Explosive Charges, and now abandoned.

The present invention relates to explosives of the contained type, and more particularly to an explosive assembly incorporating a construction for directing the major effect of the explosive and for increasing the efficiency of the blast.

it is generally well known that charges hollowed out on the side toward the material to be blasted are used for demolition purposes. The said explosive devices result in explosive waves which appear to be focussed and localized in such a manner as to cause increased penetrating effect into the adjacent surface and body. There are yielded considerably heightened penetrating effects whereas the destructive effects become smaller behind the rear front of the work or target to be blasted. It is furthermore known in the art that the penetrating power of a shaped charge is also affected by the socalled stand-off distance and that the effect of the explosive may be further increased by placing between the explosive charge and the target a tapered guiding sleeve in such a way that the smaller end of said guiding sleeve points towards the target. The presence of a guiding sleeve interferes with the perforating jet issuing from the charge when denoted, in such a manner as to direct it substantially in axial direction and at the same time to materially impair its perforating characteristics and penetrating power.

Attempts have been made for rendering the shaped charges more efficient by connecting in series. The detonation is, then, propagated frontwards. However, the expected increase in efficiency hitherto could not be obtained due to the fact that the gaseous jet of a single charge causes interference with the others.

One object of the present invention is a novel and improved explosive device and assembly whereby enhanced destructive effects are obtained.

A further object is such an assembly with which a controlled and directed blasting effect on increased effectiveness results.

A still further object is such an arrangement allowing the deep penetration of said blasting effect into any abutting material. Another object is to provide an apparatus for providing a gaseous penetrating jet in a novel and effective manner.

Yet another object is to provide a novel explosive construction for the use in the oil industry.

Other objects are to provide a novel explosive which consists of more than one charge, which preferably is combined of two different charges, one of them being a shaped-type charge, and whereby at least one guiding sleeve is provided which guiding sleeve serves to direct the gaseous jets resulting from detonation of either charge to the same target point, while the detonation of each charge is accomplished in chronological sequence, one after the other, and whereby the novel explosive-assembly may be readily constructed in accordance with known manufacturing methods, and may be of any desired size, and which is efficient in fulfilling the manifold purposes for which it is intended.

Further and other objects will appear from the detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a diagrammatical view illustrating the principle of this invention;

FIG. 2 is a diagrammatical view illustrating principally another embodiment of this invention;

FIG. 3 illustrates an arrangement suitable for measuring the effect of the combined charges; and

FIG. 4 is an enlarged cross-sectional view of an embodiment of this invention illustrating a preferred installation of the ignition cable.

Referring to the drawing more particularly by reference numerals, FIG. 1 illustrates the elementary form which the present invention may take. A hollow charge 1 is combined with a lower component of a secondary charge 2 which preferably is provided with a cylindrical bore and the front face of which is located substantially in a plane, and a guiding sleeve 3 is arranged in front of said front face and is extended between the secondary charge and the target 4.

It will be understood that in use of this explosive assembly the hollow charge 1 is to be discharged individually and earlier than the secondary charge 2. The gases developed by ignition of charge 1 pass the cylindrical bore of the secondary charge as well as the guiding conduit being formed by the guiding sleeve and penetrates into the target 4. Then the secondary charge 2 is ignited and the gases thus developed are focussed by means of the guiding sleeve 3 and are directed into the hole punched by the gaseous jet resulting from the hollow charge 1, thereby increasing the depth or the width of said hole. In case that the target 4 has already been perforated by the effect of the primary charge, then the blasting effect in the region behind the perforated zone is substantially increased. The said increased blasting effect may be made use of, with preference, when perforating oil-containing rocks, as well as military fortification and tanks.

In accordance with the invention the secondary charge is ignited at a time interval of 10 150 microseconds after the primary charge, preferably at a time interval of 40 microseconds after the primary charge.

The desired delay of time in firing the hollow charge and the secondary charge 2, respectively, may be arranged by any suitable method. For example, the desired firing order may be accomplished by means of electrical short-time igniters, the correct time interval being adjusted using means known per se, such as resistance elements, inductive elements, capacity, spark gaps, and current passages.

The delay of time in firing the secondary charge relatively to the firing moment of the hollow charge may furthermore be adjusted by means of time-lag elements and detonation transmitting drifts which are either ignited as result of the explosive reaction of the hollow charge 1 or are fired simultaneously with the latter, e.g. by means of a single countered fuse. It is to be understood that, in order to have the detonation reliably transmitted from the primary charge to the time-lag elements and from the latter to the secondary charge, blasting caps or transmission charges may be interconnected. The said detonation transmitting drifts may be lined either by wiring in the free space or by guiding in a conduit. They may likewise be provided in the form of a flat desk which is arranged onto the said secondary charge, and which is ignited by the explosive jet of the hollow charge 1.

There is a further method for adjusting the desired firing order in that the explosive effect of the hollow charge 1 is allowed to interact with a percussion cap, if desired by means of an auxiliary blasting agent. The ignition jet of the percussion cap serves to prime the detonator, or the blasting cap of the secondary cap which priming may be performed either directly or by means of a delay charge. The delay in ignition may furthermore be varied by suitably constructing the touchhole as well as the canal.

The transmission of the detonation from the hollow charge 1 to the secondary charge 2 may furthermore be effected in that a solid body which is similar to a projectile, is shot-fired by the primary explosion jet through the canal, to the said percussion cap or any other initial charge. In this modus of operation, a wide range of firing order regulation is practicable by varying the sizing of said bodies and said canals. in order to secure a far symmetrical detonation reaction in the secondary charge it may often be of advantage to multiply connect in parallel the desired transmission means instead of using a single arrangement only.

Yet another method of operation consists in affecting the firing order by means of a diaphragm.

In order to secure the correct firing order it is of advantage to avoid a direct transmission of the detonation from the hollow charge to the secondary charge, at it is, therefore, recommendable to have the secondary charge provided with a suitable cover means in order to protect it from a direct influence of the effect of the hollow charge. The cover means may be constructed as a thick-walled housing for the secondary charge. It is, moreover, preferred to arrange a further guiding sleeve 5 between the hollow charge 1 and the secondary charge 2, as illustrated in FIG. 2 which guiding sleeve 5 is of especial advantage as the hollow charge 1 and the secondary charge 2 are of equal or similar size in diameter.

The guiding sleeve 5 allows the secondary charge 2 to be safely protected against damage from fragments of the detonating hollow charge 1 and serves to more exactly directing the jet resulting from the explosion of the hollow charge 1. The said transmission means and time-lag elements may furthermore be fitted in the guiding sleeve 5.

The amount of possible increase in the effect of the blasting assembly is depending on the strength of the secondary charge, as well as on the size and strength and the resistivity and durability of the guiding sleeve. It therefore is preferred to use a metallic material, such as high-strength steel and duralumin, in fabricating the said guiding sleeve. Furthermore, there may be used any building material having a high specific gravity, in construction the guiding sleeve, since those building materials result in a more retarded delay action. It is, however, even possible to make the guiding sleeve of soft material, such as plastic or ceramic material. The increasing effect is then likewise obtained. We have found plastic material to be of especially advantage when used as building material for the guiding sleeve 5. To secure the necessary strength it is advisable not to select too small the wall thickness of the guiding sleeve, though even guiding sleeves with relatively thin walls show a remarkable effect. Sometimes a laminated structure of the guiding sleeve may cause some advantage.

It is recommendable to have the secondary charge lined, at least at its inner side which is directed towards the hollow charge jet. This holds even where there is arranged a guiding sleeve between the hollow charge and the secondary charge. Lining may be performed by inserting a short tube made of e.g. metal or plastic material, in a cylindrical bore (canal) within the secondary charge. As it is desired that the compression effect resulting from the secondary charge is as high as possible it is of advantage to provide in addition to the inner lining of the secondary charge, a jacket. The guiding sleeve 3 which points towards the target, the jacket of the secondary charge 2 as well as the casing of the hollow charge 1 can be worked in one piece. This is the preferred method of construction in case that the arrangement of the present invention is destinated to be fitted in projectiles and rockets. For reason of ballistics the shell is, as a rule, provided with a tapered nose piece, which nose piece may, according to the present invention function as the guiding sleeve 3.

The single constructed members, if not worked in one piece, may also be joint e.g. by screwing, and it will be of advantage to make the jacket of the secondary charge the internal thread of said screwing, thereby protecting the guiding sleeve from being more violently attacked than necessary, on detonation. The secondary charge may be provided with a further lining covering the surface which is directed towards the target. The shape of said lining may be flat, concave or convex, and the lining may be made of metal, such as copper, lead, and aluminum.

The canal of the secondary charge may also be constructed as a tapered bore so that the secondary charge gets the form of the lower part of a hollow charge, i.e., a form which is built when a hollow charge is cut through in vertical direction to its axis, in the region of the conical space, somewhat below the upper point of said conical space. In this case the cover of the canal of the secondary charge functions as a hollow charge lining.

In its most simple construction the secondary charge is of ring-shaped form. However, several individual charges may be arranged symmetrically round about the axis of the jet of the hllow charge. Even one single charge arranged laterally to the jet of the hollow charge results in the desired increasing and directing effect.

The explosive composition used in the secondary charge depends on the desired effect. To result in a relatively high compression power behind the target, it is advisable to use blasting agents such as dynamite, trinitrotoluene, pentaerythritol tetranitrate, cyclotrimethylenetrinitramine and mixtures thereof, if wanted in addition with substances having a high calorific intensity, such as aluminum powder. The blasting compositions can be used either compressed or cast into form. In case that an extended time effect is desired, and in particular an intensive flame formation is wanted, the secondary charge is composed of blasting agents which tend to re-burning, such as black (blasting) powder. There can be admixed substances which are only combustible with the air in the region behind the target, such as coaland metal-dust, aluminum powder and the like burning charges. It is not necessary that the secondary charge is a uniform one, it can be worked in layers, e.g. a booster may serve to blow an incendiary composition or a combustible (powder) train throughout the guiding sleeve and the perforation resulting from the hollow charges jet. Even the front-face lining of the secondary charge may be utilized for the said purpose in that it is built from sintered or pressed aluminum powder or the like substances.

Suitably, the guiding sleeves are used in the form of conical hollow bodies, however, any form which only has to be tapered in the direction towards the target, can be used for the objects of the invention. For reason of effect it may sometimes be of help to provide the guiding sleeve 3 with a cylindrical piece joint at the end which touches the blasting agent, which cylindrical piece may be provided as extended as to include the whole secondary charge.

The hollow charge can be provided in any manner well known in the art. There could be used lining which are conical in shape, or having the form of a hemisphere, or being parabolic or cylindrical in shape, or being curved. The lining angle as well as the curvature are to adjust in specific relation to the distance of the hollow charge the target, i.e., from the total height of the two guiding sleeves and the secondary charge. The thickness of the lining should be adjusted in the optimum relation to the basic width of the hollow charge and the blast load. It may be constructed progressively or degressively in thickness.

As blasting compositions for the hollow charge there are used with preference high-efficiency blasting agents, particularly blasting agents having a high speed of detonation, such as cyclotrimethylenetrinitramine, pentaerythritol tetranitrate, trinitrotoluene, and its pressed or cast blends if wanted in admixture with substances having a high calorific intensity, such as metals and oxidizing agents.

The following Examples are intended to explain the present invention but without limiting it:

EXAMPLE I The guiding sleeve 3 is made of steel and has, regarding its outer side, the form of a cylinder, the diameter of which is 3.15 inches and the height of which is 3.15 inches, too. The inner canal of the guiding sleeve 3 consists of an upper part of cylindrical shape which is approximately 0.2 inches in height and has an inside diameter of about 1.77 inches. There is the conical part connected to said upper part. The said conical part is tapered to a diameter of about 0.4 inhes, at the bottom opening.

In the cylindrical bore-hole of the guiding sleeve 3 there is inserted a ring-shaped secondary charge 2, the outside diameter of which is about 1.77 inches, while the inner height amounts to about 1.18 inches, and the outer height amounts to about 0.8 inches. Its canal is covered by means of an iron tube 6 of about 0.6 inches in inside diameter and 0.1 inches in wall thickness. The secondary charge contains 60 grams of a cast blend of trinitrotoluene RDX 1:1.

The secondary charge is covered by the guiding sleeve 5 made of steel and having an outside diameter of 3.15 inches and a height of 1.28 inches. The inside diameter at the top ranges to about 1.77 inches and the inside diameter at the lower end of the guiding sleeve 5 ranges to about 0.5 inches. The canal of the guiding sleeve 5 shows at its upper end a cylindrical part of about 0.2 inches in height. The said conical part serves to take up the hollow charge 1, which is sheathed by a casing made of an iron tube of about 1.77 inches in outside diameter, about 4 inches in height, and about 0.04 inches in wall thickness. The hollow charge 1 is lined with a conical-shaped copper-lining of about 0.04 inches in thickness and having a lining angle sized The hollow charge 1 contains 210 grams of a cast blend of trinitrotoluene RDX 1:1.

The transmission of the detonation from the hollow charge 1 to the secondary charge 2, in the right firing order, is effected as is shown in FIG. 4 by means of two detonating fuses of 1 1.8 inchesin length, respectively, interconnecting the primary charge with the secondary charge. The fuse-pieces, at its ends, are provided with a commercial detonator No. 8, each, which detonators are fitted into the charges in the manner illustrated in FIG. 4.

For testing the compression effect arising with the process and the assembly of the invention, behind the target, a cylindrical chamber 10 is made of steel, as shown in FIG. 3, is used. There is an annular member 11 made of steel, the walls of which are about 0.4 inches thick, the inside diameter of which amounts to about 60 inches, and the height of which is about 40 inches, and the member being provided with flanges, said annular member 11 is closed and screwed, at its two ends, by circular-shaped steel-

plates

12 and 13 of about 2 inches in thickness. The cover-plate 12 is provided with a circular-shaped hole 14 of about 2 inches in thickness, in the center, which hole is covered by means of a square steel-plate 15 of about 2 inches in thickness and about 12.6 inches in length of edge. The charge-assembly according to the invention is set up upon this target plate 15.

In the chamber 10 there are installed three measuring

boxes

16, 17, and 18, for testing the pressure, with 0.5 mm. lead-diaphragm, in a distance of about 122 inches from the fire-axis, and that 138 inches, 215 inches, and 300 inches, respectively, below the cover-plate. For checking the perforating power and for preserving the chamber, there is arranged, on the bottom, a stack of plates 19 being 10 inches high and about 8 inches in diameter, the individual plates of which are of about 0.1 inches in thickness.

When the hollow charge is electrically ignited, at the top, the following result is obtained:

The target-plate is perforated, furthermore 10 plates of the stack are perforated. The measuring box 16 indicates a pressure of 121 psig, the measuring box 17 indicates 136 psig, and the measuring box 18 indicates 85.3 psig. For reason of comparison, we have tested, under substantially the same conditions, a single common hollow charge, the outside construction of which was similar to that of the above described charge, but the content of blasting agent has been increased to meet with the sum of the amount of blasting agent of the said primary and secondary charge. The perforating result corresponds to that of the first testing: 13 plates of the stack are perforated, but the corresponding pressure indicated by the measuring

boxes

16, 17 and 18 amounts to only 14.2 psig, 21.4 psig, and 18 psig, respectively.

It is noted from the tests that the process of the invention results in a considerable increase in the blasting effect what is due to the combination of the two charges and the direction of the detonation products. In the EX- ample described here, the pressure efficiency behind the target is four to eight times enlarged.

EXAMPLE n Within the measuring chamber described in Example I a larger charger-assembly, as is shown in FIG. 4, is tested. The guiding sleeve 3 is a conical-shaped hollow body made of steel, having a wall thickness of 0.4 inches and a height of about 6 inches. its upper opening is of about 3.55 inches in diameter, the basic opening amounts to about 0.8 inches.

A covering frame 7 is directly applied to the guiding sleeve 3, the wall thickness of which likewise amounts to about 0.4 inches. The outside diameter is about 4.3 inches and its height amounts to about 0.6 inches. A ring-shaped secondary charge 2 is inserted into said covering frame 7, the inner edge of which amounts to 2.4 inches in height whereas the outer edge is about 1.6 inches in height. The inner side of said secondary charge is lined by an iron tube 6, the inside diameter of which is about 1.6 inches and the wall thickness of which amounts to about 0.1 inches. The blasting agent in the secondary charge 2 consists of a cast blend of trinitrotoluene-RDX 1:1, the charge contains 375 grams of said blasting agent.

Annexed to the said secondary charge there is a guiding sleeve made of steel, having an outside diameter of about 4.4 inches and a height of about 2.4 inches. The conical hollow space has an upper opening of 3.6 inches and an opening in the bottom part of 1.77 inches. Applied onto the guiding sleeve 5 there is arranged a hollow charge 1 armed with a steel casing 8 of 0.4 inches in thickness and having an outside diameter of about 0.4 inches. The hollow charge is provided with 80 grams of a cast blend constituting of trinitrotoluene RDX 1:1. The point angle of the copper lining is sized 60, the thickness is 0.1 inches.

The transmission of detonation from the primary charge to the secondary charge is performed by means of four detonating fuses, the length of which is about 24 inches each. The fuses are provided with detonating caps. The said charge-assembly is placed onto a target 4 of about 8 inches in thickness and is electrically ignited. The target 4 and in addition 32 plates of the stack 19 (see FIG. 3) are perforated. The measuring boxes which are arranged in a distance of 14 inches from the fire-axis and are provided with a lead diaphragm of 1 mm. in thickness, show the following pressure values:

measuring box 16 I49 psig measuring box I7 291 ps g measuring box 18 341 psig A comparison testing shot carried out with a single common hollow charge containing 1.255 grams of the blasting agent results in the following pressure values:

measuring box lb 533 psig measuring box 17 604 psig measuring box 18 32.0 psig.

We claim:

2. In an explosive assembly, in combination, an annular secondary explosive charge forming the forwardmost explosive charge of the assembly; tubular directing means operatively connected to said secondary charge and extending forwardly therefrom for directing the blast from said secondary charge along a given path to a given target; a primary explosive charge coaxially aligned with and located rearwardly of said secondary charge so that the blast from said primary charge will pass through said secondary charge to also be guided by said tubular directing means toward said target; and means operatively connected to both of said charges for igniting said secondary charge at a time interval of 10-150 microseconds after said primary charge whereby the blast from said secondary charge will increase the destructive effects of the blast of said primary charge.

3. In an explosive assembly, in combination, an annular secondary explosive charge forming the forwardmost explosive charge of the assembly; tubular directing means operatively connected to said secondary charge and extending forwardly therefrom for directing the blast from said secondary charge along a given path to a given target; a primary explosive charge coaxially aligned with and located rearwardly of said secondary charge so that the blast from said primary charge will pass through said secondary charge to also be guided by said tubular directing means toward said target; and means operatively connected to both of said charges for igniting said secondary charge at a time interval of 10-150 microseconds after said primary charge whereby the blast from said secondary charge will increase the destructive effects of the blast of said primary charge, said secondary charge having a cylindrical bore through which the blast of said primary charge is guided to said tubular directing means.

4. In an explosive assembly, in combination, an annular secondary explosive charge forming the forwardmost explosive charge of the assembly; tubular directing means operatively connected to and extending forwardly from said secondary charge for directing the blast thereof along a given path to a given target; a primary explosive charge located rearwardly of and coaxially aligned with said secondary charge; second directing means of substantial thickness located between and operatively engaging said primary and secondary charges for directing the blast of said primary charge through said secondary charge into said firstmentioned tubular directing means; and means operatively connected to both of said charges for igniting said secondary charge at a time interval of 10-150 microseconds after said primary charge is ignited whereby the blast from said secondary charge will enter an area which has previously been occupied by the blast of said primary charge so that the blast of said secondary charge will increase the destructive effects of the blast of said primary charge.

5. In an explosive assembly, in combination, an annular secondary explosive charge forming the forwardmost explosive charge of the assembly; tubular directing means operatively connected to and extending forwardly from said secondary charge for directing the blast thereof along a given path to a given target; a primary explosive charge located rearwardly of and coaxially aligned with said secondary charge; second directing means of substantial thickness located between and operatively engaging said primary and secondary charges for directing the blast of said primary charge through said secondary charge into said firstmentioned tubular directing means; and means operatively connected to both of said charges for igniting said secondary charge at a time interval of ll50 mirespectively.

Claims

4. In an explosive assembly, in combination, an annular secondary explosive charge forming the forwardmost explosive charge of the assembly; tubular directing means operatively connected to and extending forwardly from said secondary charge for directing the blast thereof along a given path to a given target; a primary explosive charge located rearwardly of and coaxially aligned with said secondary charge; second directinG means of substantial thickness located between and operatively engaging said primary and secondary charges for directing the blast of said primary charge through said secondary charge into said first-mentioned tubular directing means; and means operatively connected to both of said charges for igniting said secondary charge at a time interval of 10-150 microseconds after said primary charge is ignited whereby the blast from said secondary charge will enter an area which has previously been occupied by the blast of said primary charge so that the blast of said secondary charge will increase the destructive effects of the blast of said primary charge.

5. In an explosive assembly, in combination, an annular secondary explosive charge forming the forwardmost explosive charge of the assembly; tubular directing means operatively connected to and extending forwardly from said secondary charge for directing the blast thereof along a given path to a given target; a primary explosive charge located rearwardly of and coaxially aligned with said secondary charge; second directing means of substantial thickness located between and operatively engaging said primary and secondary charges for directing the blast of said primary charge through said secondary charge into said first-mentioned tubular directing means; and means operatively connected to both of said charges for igniting said secondary charge at a time interval of 10-150 microseconds after said primary charge is ignited whereby the blast from said secondary charge will enter an area which has previously been occupied by the blast of said primary charge so that the blast of said secondary charge will increase the destructive effects of the blast of said primary charge, both of said directing means having inner frustoconical surfaces and respectively having smaller ends of said inner frustoconical surfaces located closer to the target than the larger ends thereof, respectively.