WO2008020884A2

WO2008020884A2 - Internally primed cartridge case

Info

Publication number: WO2008020884A2
Application number: PCT/US2007/003216
Authority: WO
Inventors: William A. Dittrich
Original assignee: General Dynamics Ordnance And Tactical Systems-Simunition Operations, Inc.
Priority date: 2006-02-03
Filing date: 2007-02-05
Publication date: 2008-02-21

Abstract

A center-fire cartridge case and primer assembly for use in ammunition incorporates a relatively thin outer casing sidewall having an uninterrupted base at the head end of the case that is deformable in the manner similar to that of a standard.22 rimfire case. The case can include a rim or be of rimless configuration. Within the case a primer carrier having a cavity occupied by primer composition delivers igniting gases to the casing interior through a passageway in the base of the cavity within the carrier. The primer composition is positioned by the primer carrier adjacent to the interior wall of the head end. This exposes the primer composition to detonation by the deformation of the center of the head end of the case when it is struck by a firing pin. Incorporation of the primer assembly within the interior of the case reduces the possibility of leakage of gases in the area of the primer and provides a means for reducing ammunition weight.

Description

Invention Title: Internally Primed Cartridge Case

Field of the Invention

This invention pertains to the field of ordnance. It addresses a cartridge case having a one-piece, uninterrupted flat base, fully enclosing an internal primer component containing a primer explosive positioned for center-fire activation, which prevents leakage of explosive gas and simplifies manufacture of ammunition.

Background to the Invention

Centerfire cartridges used, for example in small caliber rifles and pistols, normally utilize a cartridge casing having a cavity, pocket or primer well in the center of the face surface of the base or head-end of the cartridge. This primer well is occupied by an initiating element or primer that contains an explosive primer composition.

The primer is typically in the form of a small, thin cup or "cap", usually made of brass, which contains a pressure or impact sensitive explosive material, such as lead styphnate. In the case of a Boxer primer, this primer cup also contains an anvil or metal component formed to provide a pressure point, normally at the center of the forward end of the cup. In a Berdan type primer, the anvil is not present in the cup itself but is replaced by a protrusion formed in the primer well of the cartridge case.

The primer is initiated by the impact of the firing pin in a weapon which indents the base of the primer cup and compresses the explosive between the deformed base and the internal anvil or case protrusion that serves as the anvil. This causes the primer to explode and provide the energy required initiating burning of the propellant, the output of which accelerates the projectile or performs such other action for which the cartridge was designed.

The primer is normally manufactured separately from the case and is inserted or pressed into the primer well as a separate operation. The high gas pressures experienced when the explosion is initiated require that the primer fit very securely into the case so that no or minimal leakage of gas occurs and to reduce the prospect that the primer is blown out, when initiated, complicating extraction of the spent casing. A tight seal is also desirable to prevent moisture from entering the cartridge, which could desensitize the explosive and/or propellant and prevent it from functioning.

Some smaller rimmed or rimfire cartridges, such as caliber .22 cartridges, use an explosive primer mix located internally in a cavity formed in the circumferential rim at the casing head end to initiate the main propellant charge. In such ammunition, the firing pin of the weapon strikes the outside surface of the rim, deforming it and compressing the priming explosive within the inner rim cavity, causing the explosion of the primer mix. Such cartridges are sealed at the rear and do not require the insertion of a separate primer cap. However, the area deformed by the firing pin, that is the rim, must be sufficiently thin and /or soft to permit its indentation by the firing pin. Consequently, such a rim cannot sustain the higher pressures experienced in larger sizes of ammunition, e.g. 7.62-20 millimeter ammunition. Therefore, the common ammunition of these larger sizes utilize the separate primer cap, located centrally in the primer well at the head end of a cartridge, the head end being thick and strong enough to withstand the pressures experienced during the action of firing a round.

A US patent, 268,601 to Bailey issued in 1882 depicts an internally primed cartridge casing wherein the head end is built-up from a number of components that includes an outer metal "cup" or extension from the head end that reinforces the cylindrical side wall of the cartridge casing, and an inner iron retaining disk that seats the primer well in place. The primer composition is placed in a central location against the interior face of the head end, contained within two interfitting cups held against the head end by the separate iron retaining disk and surrounded by paper packing.

More recently, three United States patents, US 6,976,431, US 6,959,647 and US 6,367,389 similarly address internally primed cartridge casings. Figure 7 in US 6,367,389 to Westrom depicts primer composition deposited in a central well integrally formed on the interior face of the head end. Sidewalls of this well are crimped-over to retain the composition in place and provide the function of an anvil.

Both US 6,959,647 to Reynolds et al and US 6,976,431 to Westrom et al depict a number of configurations wherein primer composition is deposited on the interior face of the head end of a cartridge case in an annular layout. As well examples are given wherein the primer composition is centrally located. In virtually all examples, with the exception of Figure 11 in both of these references, the primer composition is apparently delivered as such to the casing interior for deposit at its final location within the casing.

In respect of Figure 11, the primer composition is contained within the casing in a cup-shaped receptacle 229 that is open on its forward end. It is stated that the primer composition can be pre-placed in this receptacle. A separate anvil, 223 (not shown in Figure 11), is then placed over the priming composition within its receptacle. A central opening 227 in the head end face of the casing provides access for a firing pin to strike the base of the cup- shaped receptacle. Thus, even in this case, the containment of the primer composition within the head end is built-up from a number of components It is an object of this invention to combine the function of a center fire primer with sealed head end for of a cartridge case as in small rim-fire cartridges, while providing the strength and centrally located primer common to the larger and higher pressure ammunitions. It is also an object of the invention, amongst others, to simplify the manufacturing process for cartridges as by the use of thinner input materials, such as stainless steel strip for manufacture.

The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.

Summary of the Invention

According to one aspect of the invention, in order to provide a center- fire cartridge case with a primer assembly, a primer carrier having a cavity occupied by primer composition is located adjacent to the interior wall of the head end. A passageway in the base of the cavity within the carrier provides access for igniting primer gases to initiate propellant present in the casing interior. The carrier is positioned so that open end of the carrier cavity, with primer composition present therein lies against the interior wall of the head end. This exposes the primer composition to detonation by the deformation of the center of the deformable head end of the case when it is struck in its center by a firing pin. The head end of the case can thereby be completely closed, improving the retention of gases within the interior volume of the case on firing. Incorporation of the primer assembly within the interior of the case reduces the possibility of leakage of gases in the area of the primer and provides a means for reducing ammunition weight and cost. The invention also provides a simplified and improved method for manufacturing ammunition.

According to the present invention in another aspect, a center-fire cartridge case is provided which comprises:

a) an outer case having casing sidewalls surrounding a case interior and having a closed head end which is centrally deformable by a firing pin,

b) an inner primer carrier with a central cavity having a base end and an open- end, the carrier serving for containing a primer composition and having a passageway penetrating through its base end for the propagation of the explosive products of a primer composition upon detonation into the case interior to ignite propellant when present therein,

wherein the carrier is fitted within the case interior with its open end directed towards and adjacent to the interior face of the head end of the case. The case can include a rim or be of rimless configuration.

An advantage of this configuration is that the primer composition can be preloaded into the carrier and then delivered into position within the casing interior by depositing the carrier at the head end of the case. In order to retain the primer composition in place during this operation, a penetratable primer containment barrier is placed over the open end of the carrier cavity. This primer containment barrier may be in the form of a polymeric sheet, such as Mylar (trademark) polyester film or other equivalent means including aluminum foil. Additionally, the passageways through the base end of the cavity may be sealed by a rupturable barrier similar to the primer containment barrier. Such rupturable barrier can serve to protect the primer composition from exposure to moisture and other contaminants originating within the case interior.

It is highly desirable that the carrier include an anvil positioned at the base end of the cavity. This may be provided in the form of an insert seated at the base end of the cavity, e.g. a cone or disk with a nib. Or it may be in the form of one or more protrusions extending up from the base of the cavity and directed towards the open end of the cavity. Such protrusions may be located adjacent to the passageway(s) formed in the base of the cavity to vent primer gases into the case interior. Such protrusions may be formed from a portion of the base end of the cavity that is deflected into the cavity in order to provide the venting passageway.

The primer carrier of the invention may be conveniently combined with a thin-walled cartridge case particularly of the rimless type that would otherwise be vulnerable to blowout around the periphery of the head end in the region of the extraction groove. This may be achieved by shaping the outer circumferential surface of the carrier to lie against and reinforce the casing sidewall. Preferably, this is a sealing contact. In a preferred variant, the primer carrier includes a solid portion that extends between the cavity formed therein and the casing sidewall.

Additionally, the primer carrier can have a shaped surface, on the side opposite to the open end of the cavity that is dished-up in the form of an annular rim to provide further support to the casing sidewall in the direction towards the open end of the casing. Again, this forward extension to the carrier can be tapered with a reducing thickness that will allow its peripheral end to expand slightly when the cartridge is fired. This expansion can improve the sealing effect between the carrier and the casing sidewall. An improved seal will contribute towards permitting use of a case with a thinner sidewall by protecting the sidewall at the head end from being blown-out when the cartridge is fired.

According to a method for assembling a primer composition within a center-fire cartridge case the following steps may be followed:

a) providing an outer case having casing sidewalls surrounding a casing interior and having a closed head end which is centrally deformable by a firing pin,

b) inserting into the casing interior a primer carrier, the carrier having a central cavity with a base end and an open-end, and a passageway penetrating through the base end of the cavity for the propagation of the explosive products of a primer composition upon its detonation, the carrier containing a primer composition retained within the cavity, optionally by a penetratable primer containment barrier overlying the open end of the cavity, and

c) seating the primer carrier within the case with the carrier positioned against the head end of the case, the open end of the cavity being directed towards the head end of the case and the primer composition being located adjacent to the head end.

The primer carrier may be retained in place by providing a jam- fit between the carrier and the casing sidewalls, by incorporating an adhesive at this interface or by other known means. Optionally, fabrication of the case can include the further step of forming in the casing sidewall an inwardly directed rib positioned to retain the carrier in position within the casing. 4.

According to a further feature of the invention a central portion of the head end corresponding to the location of the primer composition within the interior of the case may be of reduced thickness to facilitate the deformation of the face of the head end by a center- fire firing pin.

In addition to providing increased protection from leakage in the area of the primer as compared to conventional primed centerfire cases, this invention provides a method for making a lighter weight case which may prove more advantageous to produce.

The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow.

Brief Description of the Drawings

Figure 1 is a cross-sectional side view of a rimmed cartridge case having a primer carrier insert formed from a sheet of metal positioned against the interior face of the head end of the case.

Figure 2 is a cross-sectional side view of the primer carrier of Figure 1.

Figure 3 is a cross-sectional side view of a variant on the primer carrier of Figure 2 wherein an anvil in the form of a nib is provided by deflecting a portion of the carrier wall inwardly while forming a gas venting passageway.

Figure 4 is a cross-sectional side view of a variant on the primer carrier of Figure 2 wherein the carrier has a carrier rim to fit within the rim of the case.

Figure 5 is a cross-sectional side view of a rimmed cartridge case having a primer carrier insert formed from a solid block positioned against the interior face of the head end of the case. Figure 6 is a cross-sectional side view of the primer carrier of Figure 4.

Figure 7 is a cross-sectional side view of a variant on the primer carrier of Figure 6 wherein an anvil is provided in the form of an insert shaped as a conical disk.

Figure 8 is a cross-sectional side view of a variant on the primer carrier of Figure 6 wherein the carrier has a carrier rim to fit within the rim of the case.

Figure 9 is a cross-sectional side view of a rimless cartridge case within an extraction groove having a solid primer carrier insert positioned against the interior face of the head end of the case.

Figure 10 is a cross-sectional side view of a variant of the primer carrier of Figure 9 wherein the carrier has a tapered annular rim lying against and extending forwardly along the casing sidewall.

Figure 11 is a cross-sectional side view of a close-up of the head end of a variant on the casing of Figure 9 wherein the central portion of the face of the head end is of reduced thickness.

Figure 12 is a cross-sectional side view of a cartridge containing propellant and having a projectile fitted into the case of Figure 9. The cartridges shown loaded into the chamber of a barrel with a bolt and firing pin 34 positioned against the head end of the caste for firing.

Description Of The Preferred Embodiment

In Figure 1 a case 1 with a head end 2 is provided with a casing sidewall

3 and a case interior volume 4. Within the case interior 4 a primer carrier 5, having a cavity 6 with one open end 9 and filled with primer composition 7, is positioned adjacent to the interior wall of the head end 2. Primer composition 7 present within the cavity 6 preferably lies against or proximate to the interior wall of the head end 2 to which it is directed at the open end 9 of cavity 6.

The outer casing sidewall 3 and head end 2 if made of metal, may be formed from thin sheet steel, aluminum, brass or similar alloys, in the manner of a smaller cal.22 cartridge case, using, for example progressive die press systems common in the stamping/drawing industry. The head end 2 is sealed and the material of the head end 2 is deformable so that, when struck in its center by a firing pin, the primer composition 7 will explode. The resulting component should have a head end 2 thickness and metal properties in the center of the head end 2 base area similar to that of a primer cup in a conventional ammunition, allowing it to deform inward, when struck by the firing pin of a gun or similar device.

A passageway 8 in the base of the cavity 6 within the carrier 5 provides access for igniting gases from the primer composition 7 to initiate propellant (not shown) present in the case interior 4. The volume of the cavity 6 is sufficient to provide a little free-space when filled with primer composition 7. This, combined with providing vent opening passageways 12 in the cavity base end of sufficient diameter can serve to limit the development of excessive overpressure is in the cavity 6 when the primer composition 7 detonates.

In order to retain the primer composition 7 in place, a penetratable primer containment barrier in the form of a polymeric sheet, such as a Mylar (trademark) polyester film, or aluminum foil 8 is placed over the open end 9 of the carrier cavity 6, as shown in Figure 2. This is an alternative to employing an adhesive, compatible with the function of the primer, to bind the primer composition 7 together and in place within the cavity 6. The carrier 5 of Figure 1 is in the form of a deformed sheet, preferably sheet metal. In this embodiment the carrier 5 has an annular carrier sidewall 13 that lies against the interior surface of the casing sidewall 3, protecting the casing sidewall 3 from bursting under the high pressure arising upon ignition of the propellant.

The carrier 5 as shown in Figure 2 is provided with an anvil 10 positioned at the base end of the cavity 7. An anvil 10 serves to focus the compression stresses in the priming explosive when impacted by the firing pin or other initiating means. An anvil 10 may be in the form of a hump raised in the base end of the carrier 5, shaped in the manner of the anvil as in standard boxer primed ammunition. This configuration is shown in Figure 2. Or an anvil 10 may be in the form of one or more protrusions 11 erected out-of the base end of the carrier 5, as shown in Figure 3. Alternately an anvil 10 may be in the form of "nibs" 1OA extending up from the base of the cavity 7 and directed towards the open end 9 of the cavity 7, as shown in Figure 1.

Such protrusions 11 may optionally be formed from a portion of the base end of the cavity 7 that is deflected into the cavity 7 to also serve to provide the venting passageway 12 as shown in Figure 3. Thus the function of the protrusion and the venthole 12 may be combined wherein the passageway 12 is formed by punching partially through the base end of the cavity 6, bending the material that had been in the location of the passage 12 into the shape of a protrusion 1 1.

Adjacent the hump 10, nibs 1OA or protrusions 11 are one or more passageways 12 formed in the base of the cavity 7 to vent primer gases into the case interior 4. The passageways 12 through the base end of the cavity may also be sealed, as shown in Figures 2 and 6, by a rupturable barrier 14 similar to the primer containment barrier 8 in order to protect the primer composition 7 from exposure to moisture and other contaminants originating within the case interior 4.

The primed case 1 functions in the same manner as a conventional cartridge case. The sensitivity to firing pin impact may be controlled by the thickness and material properties of the head end 2 at the point of impact, the configuration of the anvil protrusions 10, 11 and the characteristics of the priming explosive 7. When the center of the head end 2 is struck by a firing pin (not shown), the impact deforms the outer face of the head end 2, compressing the priming mixture 7 between it and the anvil 10 to detonate the primer.

Figure 5 shows a primer carrier 5 A which is of solid form. In addition to providing a cavity 6 for the priming explosive 7, it may also provide reinforcement to the casing sidewall 3 around the head end 2 area of the casing 1 to permit the use of a thin- walled case 1 in high-pressure environments. Such a carrier 5 A may be made of brass, copper or other suitable material which will provide protection for the portion of the casing sidewall 3 that could otherwise be vulnerable to blowout on firing of the cartridge. As a solid part, the carrier 5A may be formed from metal using cold heading or stamping processes or it may be cast from metal or polymeric material.

The variant carrier 5B of Figure 7 has a single passageway 12 and an insert in the form of a conical disk 15 having central peak 16 that serves as an anvil. This peak 16 is an alternate to providing an insert in the form of a circular disk that has a nib 1OA or protrusion 11. This conical disk 15 may also be penetrated by one or more passageways 12A to vent primer gases.

In Figure 8 the carrier 5 A of Figure 6 has been modified to add a slight annular rim 16 around the edge of the carrier 5C of Figure 8. This carrier rim 16 is located on the carrier 5C for positioning adjacent to the head end 2. This rim 16 is useful in cases where the final rim 17 on the case 1 is not fully formed when the carrier 5C is inserted therein. A final forming action further deforming the head end 2 to produce the final casing rim 17 may then engage with the slight annular carrier rim 16 to lock the carrier 5C in place. This feature is particularly suitable for use with rimless ammunition, in which case the carrier rim 16 will engage with a portion of the casing sidewall 3 A that is deformed inwardly to provide an extraction groove 18, as in Figure 9.

The method of assembly is as follows. The outer shell for the casing 1 is formed as a straight tube with a flat base. After the rimmed carrier 5C is dropped in, the outer casing sidewall 3 is rolled or crimped to form the extractor groove, or in the case of a rimmed case, the entire sidewall 3 is crimped or folded in to provide the desired shape that includes overlying the carrier rim 16 to the primer carrier 5C in place.

In Figure 9, a solid carrier 5D has been fitted into a case IA which is rimless. A penetratable primer containment barrier 8 contains the primer composition 7 within the carrier cavity 6. This case IA incorporates an extraction groove 18 positioned just inside the casing rim 17. The carrier 5D of Figure 9 may also, optionally, incorporate a slight annular rim 16A similar to that shown in Figure 8 in order to retain the carrier 5D seated against the head end. Additionally, a crimped ring or circular groove 38 is formed in the sidewall to provide an inner rib 39 adjacent to the forward end 21 of the carrier 5D. This rib 39, as well as the annular carrier rim 16A, retain the carrier 5D again shifting forward within the case 1 The outer circumferential surface of the carrier 5D lies against and reinforces the casing sidewall 3A with a sealing contact. As with the variant of Figure 5, the primer carrier 5D includes a solid portion that extends between the cavity 6 formed therein and the casing sidewall 3A.

Additionally, the primer carrier 5D can have a forward extension with a shaped inner surface 19 as shown in Figure 10 that is dished-up to provide a forward annular rim 20 as further support to the casing sidewall 3A. The forward annular rim 20 is preferably tapered with a reducing thickness that will allow its peripheral, forward end 21, combined with the ductility of the material of the carrier 5D, to tend to expand slightly when the cartridge is fired, thereby improving the seal between the carrier 5D and the casing sidewall 3A.

In Figure 11 a central portion 23 of the head end 2 corresponding to the location of the primer composition 7 within the interior of the case 1 may be of reduced thickness to facilitate the deformation of the face of the head end 2 by a center- fire firing pin. Thus the face of the head end 2, instead of being of constant thickness across its width as depicted previously, may have an outer annular portion of a first thickness and an interior region of lesser thickness within the outer annular portion.

In Figure 12, the casing of Figure 9 forms part of a cartridge 35 having a projectile 31 with the interior cavity 4 of the casing 1 containing propellant 32. The cartridge 35 is fitted into the chamber of a barrel 36 for firing. A bolt 33 with a firing pin 34 is positioned against the head end of the casing 1. The firing pin 34 is aligned in to penetrate the central region of the head end 2, compressing the primer composition 7 against the nib 1OA in order to fire the round.

In manufacture, the carrier 5, 5A, 5B, 5C, 5D is conveniently formed separately from the case 1. The primer composition 7 is compressed into cavity 6 and a thin containment barrier 8 is applied to keep the primer composition 7 in place during assembly. The loaded carrier 5, 5A, 5B, 5C, 5D is then inserted or pressed into the case 1 all the way through to the head end 2. Thereafter any additional forming operations needed, such as crimping the rim, tapering or necking the casing sidewall 3, 3A are performed to finish the primed cartridge case 1. The case 1 is then loaded with propellant 25 and other operations performed as in normal cartridge manufacture. CONCLUSION

The foregoing has constituted a description of specific embodiments showing how the invention may be applied and put into use. These embodiments are only exemplary. The invention in its broadest, and more specific aspects, is further described and defined in the claims which now follow.

These claims, and the language used therein, are to be understood in terms of the variants of the invention which have been described. They are not to be restricted to such variants, but are to be read as covering the full scope of the invention as is implicit within the invention and the disclosure that has been provided herein.

While the present invention has been described with respect to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that other variations and modifications can be effected within the scope and spirit of the invention.

Claims

The embodiments of the invention in which an exclusive privilege is claimed are as follows.

1. A center-fired cartridge case having a base end and a forward end comprising:

wherein the carrier is fitted within the case interior with its open end directed towards the head end of the case.

2 The case as in claim 1 comprising an anvil positioned at the base end of the cavity and directed towards the open end of the carrier.

3. The case as in claim 2 were in the anvil comprises a plurality of protrusions directed towards the open end of the carrier of located adjacent to the passageway.

4. The case as in any one of claims 2 or 3 comprising an insert in the form of a disk positioned at the base and of the cavity, wherein the disc carries the anvil.

5. The case as in any one of claims 1, 2, 3 or 4 comprising a primer composition located within the cavity and a penetratable primer containment barrier overlying the open end of the cavity to retain the primer composition within the cavity.

6. The case as in claim 5 wherein the primer containment barrier comprises a thin sheet selected from the group consisting of a polymeric film or an aluminum foil.

7. The case as in any one of claims 1 to 6 wherein the carrier has an outer circumferentially surface that lies against and reinforces the casing sidewall with a sealing contact.

8. The case as in claim 7 wherein carrier includes a solid portion that extends between the cavity formed therein and the casing sidewall.

9. The case as in claim 8 wherein the primer carrier has an annular forward extension protruding away from the head end of the case, such forward extension terminating in a peripheral end and being in the form of a surface that is dished-up to provide support to the casing sidewall in the direction towards the forward end of the casing.

10. The case as in claim 9 wherein the forward extension to the carrier is tapered with a reducing thickness that, combined with the ductility of the material of the carrier, will allow the peripheral end to expand slightly when the cartridge is fired sufficiently to ensure a seal against the casing sidewall that will exclude propellant gases from bursting the casing sidewall.

11. The case as in any one of claims 1 to 10 wherein the carrier comprises an annular rim located on the carrier for positioning adjacent to the head end and engagement with the casing sidewall at a location interior to a casing rim formed on the case.

12. The case as in any one of claims 1 to 11 wherein the closed head end has a face of given width and the head end is of constant thickness across the width of its face.

13. The case as in any one of claims 1 to 11 wherein the closed head end has a face of given width and the head end is of a first thickness around and outer annular portion of its face, and is of lesser thickness within an interior region within the outer annular portion.

14. The case as in any one of claims 7 to 13 comprising a primer composition contained within the cavity of the carrier.

15. The case as in any one of claims 1 to 14 comprising a propellant within the interior volume of the case.

16. The case as in any one of claims 1 to 15 wherein the case is for use as rimless ammunition and comprises an extraction groove.

17. A method for assembling a primer composition within a center-fire cartridge case comprising:

b) inserting into the casing interior a primer carrier, the carrier containing primer composition within a central cavity which has a base end and an open- end, the carrier having a passageway penetrating through the base end of the cavity for the propagation of the explosive products of a primer composition upon detonation of the primer composition, and c) seating the primer carrier within the case with the carrier positioned against the head end of the case, the open end of the cavity being directed towards the head end of the case and the primer containment barrier being located adjacent to the head end to expose the primer composition to the inside surface of the head end

to provide a cartridge case with a sealed head end.

18. The method of claim 17 wherein the primer composition is retained within the cavity by a penetratable primer containment barrier overlying the open end of the cavity.

19. A method as in any one of claims 17 or 18 including the further step of forming in the casing sidewall a shoulder towards the forward end of the casing to provide a length of casing sidewall of reduced diameter.