- 1 PROJECTILE FOR USE IN A BARREL WITH A PLURALITY OF STACKED PROJECTILES FIELD OF THE INVENTION [0001] This invention relates to systems for sealing of propellant charges in relation to stackable projectiles, particularly to a system for sealing of a propellant charge inside a projectile to prevent ignition of the charge by gases resulting from ignition of the leading projectiles in the stack. More particularly the invention relates to projectiles which may be loaded into a barrel assembly in the field. BACKGROUND OF THE INVENTION [0002] A wide range of sealing systems have been developed for weapons having stacked projectile arrangements or barrel assemblies, such as the "wedging" systems described in WO 94/20809 and WO 97/04281, and the "projectile-to-projectile" sealing arrangements which in WO 03/089871, for example. The projectiles in these weapons are generally caseless and temporary seals are therefore required to prevent blow-back of ignition gases down the barrel. If no sealing system is present, hot pressurised gases from ignition of a leading projectile in a stack will usually cause uncontrolled ignition of the propellant in a trailing projectile. [0003] Wedging systems generally form seals by interaction between successive projectiles in a stack. An axial force down the barrel causes the interaction either when the stack is loaded in a barrel or when projectiles are fired from the barrel, or both. The interaction causes a collar or tail on each projectile to expand into tight contact with the bore of the barrel, preventing blow-back past that point. Depending on the pressures involved, the expanding part of each projectile is typically a soft metal or plastic which deforms into a circumferential contact with the barrel. Various "forward", "reverse", "nose-to-tail" and "stick" systems have been developed. [0004] Weapons that use wedging systems can be difficult for a user in the field to reload and generally require loading in a factory or other specialised environment. A large force is usually required to form the seal and the surfaces that interact within the barrel must be sufficiently clean. Special tools may be required. Subsequent shocks or vibration may weaken -2 [0005] the seals and reduce the reliability of the weapons. Long cartridges containing pre stacked projectiles are used for reloading in the field, but when partially empty these may be problematic for the user. [0006] Systems that utilise projectile-to-projectile sealing form seals by interaction between successive projectiles. These also are not generally suitable for reloading in the field. SUMMARY OF THE INVENTION [0007] It is therefore an object of the invention to provide an improved sealing system for stacked projectiles, or at least to provide an alternative to existing systems. [0008] In one aspect the invention may be said to reside in a projectile for use in a barrel with stacked projectiles, including: a chamber containing a propellant charge for the projectile, an exit from the chamber for release of propulsion gases into the barrel when the propellant is ignited, and a seal blocking the exit which is opened by ignition of the propellant within the chamber but is resistant to gases produced by ignition of propellant in other projectiles in the barrel. [0009] In one embodiment the exit is an aperture in a wall of the chamber and the seal is a moveable barrier in the aperture, such as a valve-like structure. In another embodiment the exit is an aperture in a wall of the chamber and the seal is a rupturable barrier across the aperture. In a further embodiment the seal is a deformable barrier across the aperture, In a still further embodiment the seal is a thin barrier around the charge such as a bag, wrapping or coating and the exit involves a disintegrable character of the barrier. In a further embodiment the seal is an inherent property of the geometry of thee chamber. [0010] Preferably the seal not only resists gases produced by ignition of other projectiles in the barrel, but the action of the seal is also enhanced by the pressure of the gases. In the case of a seal formed by a moveable barrier for example, the gas pressure may urge the barrier into still closer contact with adjacent parts of the chamber. [0011] Preferably the opening of the seal in a projectile does not create debris which might impede the passage of subsequent projectiles inside the barrel. In the case of a seal formed by a rupturable barrier for example, the ruptured portions of the barrier remain attached to the chamber and are carried out of the barrel by the projectile. In the case of a seal having a -3 disintegrable character, the seal should be largely or entirely destroyed or consumed when the propellant inside the chamber is ignited. [0012] In another aspect the invention resides in a sealing system for a propellant charge, including: a container for the charge, and exit means for release of combustion gas from the container when the charge is ignited, wherein the exit means is opened by ignition of the charge within the chamber but is resistant to ignition of charges outside the container. [0013] Preferably the container is a chamber formed in a larger structure such as a projectile or barrel assembly. The exit means is typically an aperture that is closed by a moveable, rupturable or deformable barrier. Alternatively the container may be a relatively thin barrier around the charge such as a bag or wrapping, and the exit means includes rupture, burning or other disintegration of the barrier. The sealing may also be an inherent property of the chamber. [0014] The invention also resides in a barrel assembly containing stacked projectiles with independent sealing as defined above, and in methods of loading and firing projectiles having sealing systems as indicated above. [0015] These sealing systems can function to isolate propellant charges independently of other sealing interactions between adjacent projectiles or between projectiles and the barrel. A sealing action of this kind will assist the design of stacked weapons which are individually reloadable. [0016] The invention also resides in any alternative combination of features that are indicated in this specification. All equivalents of these features are deemed to be included whether or not explicitly set out. LIST OF FIGURES [0017] Preferred embodiments of the invention will be described with respect to the accompanying drawings, of which: Figure 1 shows a stackable projectile having a generalised burner system, Figures 2a, 2b show how propellant gases typically flow in a barrel when a stacked projectile is fired, Figures 3a-d show a burner system with a moveable seal, Figures 4a-d show a variation on the burner in Figure 3, -4 Figures 4e-f show a further variation, Figures 5a, b show a further variation on the burner in Figure 3, Figures 6a-c show a further burner with a moveable seal, Figures 6d-f show a further variation, Figures 7a-c show a burner system with a pivoting seal, Figures 8a-d show a burner with a rupturable seal, Figures 9a, b show a variation of the burner in Figure 8a-d, Figures 9c, d show a further variation, Figures 10a, b show a further burner with a rupturable seal, Figures 11 a, b show a variation on the burner in Figure 10, Figures 12a, b show a further burner with a rupturable seal, Figures 13a-c show a further burner with a rupturable seal, Figures 14a, b show rupture details for Figures 13a-c, Figures 15a, b show a burner with a consumable seal, Figures 16a, b show a burner with a deformable seal, Figures 17a, b, c show a burner with a moveable seal, Figures 18a, b show a burner with a deformable seal, Figures 19a, b show a burner with a deformable seal, Figures 20a, b show a burner with a deformable seal, Figures 20c, d show a projectile with the burner in Figures 20a, b, Figures 21a, b show a burner with a rupturable seal, Figures 22a, b, c show a burner with a deformable seal, Figures 23a, b show a projectile with the burner of Figures 22a,b, Figure 24 shows a tailpiece including a rupturable seal, Figure 25 shows an alternative projectile, and Figures 26a, b show stacking of the projectile in figure 25. DESCRIPTION OF THE PREFERRED EMBODIMENTS [0018] Referring to the drawings it will be appreciated that the invention may be implemented in a range of different ways for a range of different projectiles and barrel assemblies. These embodiments are given by way of example only. Systems related to the weapon which fires the projectiles will appreciated by a skilled person and need not be described in detail.
-5 [0019] Figure 1 shows a typical projectile for a stacked projectile weapon, in a cross sectional exploded form. The projectile includes a payload container 10, such as a warhead, a propellant charge 11, and a tail assembly 12. Primer 13 activates the warhead and primer 14 ignites the propellant. The projectile is adapted to be stackable nose to tail with a number of identical projectiles in the barrel of the weapon. Nose portion 15 has a roughly convex outer surface shaped to correspond with a roughly concave inside surface of the tail assembly. Various other features may also be provided, such as driving bands which improve the efficiency of firing, and a system for connecting the projectiles together. [0020] Because the projectile in Figure 1 is to be used in a stack the propellant must be sealed against ignition gases which fill the barrel of the weapon after each projectile is fired. In this example the propellant is sealed within a burner or casing 17 which is resistant to the ignition gases produced by other projectiles. The casing provides a chamber and typically includes a seal portion which moves, ruptures, deforms, disintegrates or otherwise opens under the higher pressures inside the casing which are produced when propellant 11 is ignited. However, the seal is either unaffected or is enhanced by an increase in pressure outside the casing. A range of other systems such as wedge sealing between projectile and barrel, or between nose and tail of adjacent projectiles, may be employed in addition to the internal casing system. [0020] In Figure 1 the projectile is fired from the weapon by way of an inductive system having an inductor 18 which interacts with a corresponding inductor in the barrel, and a signal detector 19 which receives output from the inductor 18 and determines whether the projectile is required to fire. The detector is typically programmed with a code and on receiving a signal containing the code from the inductor, the detector triggers the primer 14 to ignite the propellant. The detector may also arm the warhead and enable primer 13. Otherwise the detector generally remains idle. Firing systems of this kind are known and need not be described in detail. A range of other electrical or mechanical firing systems are also possible for stacked projectile weapons. [0021] Figures 2a, b indicate how propellant gases are typically distributed in the barrel of a stacked projectile weapon, particularly a weapon which is designed to be reloaded or unloaded in the field. Tolerances between the projectiles and the bore of the barrel are generally large enough to enable a sliding fit of projectiles into the bore. Projectiles 20 and 21 are leading and trailing projectiles respectively, stacked nose to tail in barrel 22. Inductors 23 -6 outside the barrel interact with corresponding inductors in the projectiles to initiate the firing process. A breech plug 24 supports projectile 21 at the base of the stack. The projectiles fit closely within the barrel, and usually include driving bands, but there is generally enough tolerance within the bore of the barrel for hot, high pressure propellant gas from a leading projectile to circulate past trailing projectiles when the leading projectile is fired. In Figure 2b the gas (shaded) from ignition of propellant in the burner of projectile 20 blows backwards down the barrel past the body of projectile 21 and reaches the outside surface of burner in projectile 21. Without sealing, there is a tendency for ignition of the propellant in projectile 21. [0022] Figures 3a-d shows a burner system suitable for use as the burner 17 in Figure 1, in order to provide sealing against ignition of the propellant by other projectiles in a stack. The burner includes a generally cylindrical casing 30 and moveable slab 31 which encase the propellant. Exit vents 32 around the casing are normally blocked by the slab and prevent ignition gases produced by other projectiles from entering the casing. The slab has an edge face 35 which abuts a corresponding face 36 inside the casing 30 to assist the seal. An increased pressure caused by gases outside the casing serves to compress the faces 35 and 36 together more closely. A spring 33 and retainer 34 hold the slab in place within the casing as shown in Figure 3b. When fired, the spring is compressed or crushed by the slab and the ignition gases produced within the burner are able to escape, as shown in Figure 3c. The projectile is then propelled by gas pressure within the barrel. [0023] A section though a coil spring 33 is shown in Figure 3d is shown as an example, although other spring types such as a disc spring or belleville washer could be suitable. Gases outside the casing 30 are able to more freely through the spring. [0024] Figures 4a-d show a variation on the burner system in Figures 3a-d. The system now includes a crush ring 40 which prevents the slab 31 from compressing the spring 33 until a predetermined pressure has been reached inside the casing. This ensures that on ignition of the propellant inside the casing, the resulting gases burn cleanly and are not released into the barrel to propel the projectile until the predetermined pressure has been reached. The ring 40 may take a range of structures and operate in a range of different ways. Figure 4d shows a circular grill structure which contains the spring 33 and allows throughflow of gas, by way of example.
-7 [0025] Figures 4e-g show a further variation on the burner system in Figures 3a-d. The system now includes a sprung disc 45 such as a belleville washer between slab 31 and the retaining ring 34. In this example, a second disc 46 has also been included with an orientation which is inverted relative to disc 45. As in Figures 4a-d the discs compress the slab inwards to form a seal with the casing until a predetermined pressure has been reached inside the casing. Figure 4g shows the burner after ignition of the propellant and opening of the seal. The discs 45, 46 have been crushed into a flat configuration. [0026] Figures 5 a, b show a further variation on the burner system in Figures 3a-d. Slab 31 in Figure 3a takes the form a disc with a bevelled edge which abuts a corresponding surface of the casing, effectively forming a wedge. Slab 51 is a simple disc shape without the bevel. Both slabs seal against a flange inside the casing to prevent entry of gases from the barrel and the greater the external pressure the stronger the sealing action. The slab 51 in Figure 5a is considered to be less effective in forming a seal with the casing than the slab 31 in Figure 3a. Figure 5b shows a series of underside views of the casing with the slab 51, a crush ring 40 and retainer 34 in place. [0027] Figures 6a-c show a further alternative to the burner system in Figures 3a-d. In this system the casing 60 contains a panel 61 with two or more vents 62. A moveable slab 63 includes corresponding keys 64 which occupy the vents and seal propellant inside the casing. A crush ring 65, spring 66 and retainer disc 61 are provided as before. External pressure caused by ignition gases outside the casing urges the keys further into the vents to improve the sealing action. On ignition of propellant inside the casing, the keys are forced out of the vents and the slab compresses the ring 65 and spring 66. Figures 6b and 6c show the casing before and after firing of the propellant respectively. [0028] Figures 6d-f show a further alternative burner system. In this system a seal with the casing is provided by a sprung disc 67, typically a belleville washer, located on a slab 68 which is typically threaded into the casing. The edges of the disc abut the casing to prevent flow of external ignition gases into the casing 60 through vents 69. The crush resistance of the disc is calculated to provide a predetermined internal pressure at which the disc is distorted and ignition gases produced inside the casing are released. [0029] Figures 7a-c show a further alternative burner having a moveable seal, suitable for use as the burner 17 in Figure 1. In this example the burner has a casing 70 and a moveable seal -8 71 in a flower form having leaves 72. Gas pressure outside the burner serves to maintain the leaves together while ignition of propellant inside forces the leaves to open. Once again the stronger the gas pressure outside the casing 70 the stronger the sealing action of the leaves. A range of different valve seals of this general kind may be envisaged. Figures 7b and 7c show the leaves in an open position. [0030] Figures 8a-d show an alternative burner having a rupturable seal, also suitable for use as the burner 17 in Figure 1 to resist blow-back of external propellant gases. The burner includes a generally cylindrical casing 80 which contains propellant, and a series of metal discs which form a closure for the casing. Vent disc 81 includes four vents 82 while burst disc 83 includes corresponding sealing portions 84 which cover the vents. A retainer ring 85 holds the discs within the casing and provides opening or openings 86 to allow the burst disc to operate. Vent disc 81 may be omitted whereby the burst disc 83 seats against the shoulder in the casing 80. Retainer 85 may threadably engage the inner wall of the casing and sealingly clamp the vent disc 81 or burst disc 83 against the shoulder. Pressure from ignition gases external to the casing is reduced by the overall volume available in the barrel, and is resisted by the burst disc 83. However, pressure caused by combustion of the propellant inside the casing causes the disc 83 to rupture, releasing gases which propel the respective projectile. The burst disc 83 is scored or otherwise constructed in a way which ruptures in a predictable fashion, generally at or above a predetermined pressure and/or temperature, and leaves no significant debris in the barrel of the weapon. Figure 8d shows several scoring patterns, by way of example. [0031] Figures 9a, b show a variation on the rupturable burner in Figures 8a-d. In this example, the vent disc 90 includes a single central aperture 91 as the vent. The remaining components are substantially similar to the previous example. Burst disc 83 and retainer 85 are provided as a seal over the vent disc with the retainer typically being threaded into the casing to hold the burst disc in place. Various structures of this kind are envisaged to enable accurate tailoring of the burner system to suit particular projectile types and environments. [0032] Figures 9c-d show a further variation on the rupturable burner in Figures 8a-d. As described above for Fig 8, the vent disc 90 may be omitted. In this system the retainer 95 cooperates with the burst disc 96 to reduce the likelihood that debris will be left in the barrel after the respective projectile has been fired. The retainer takes the form of an annulus or ring as before, but the inner edge 97 of the ring is sloped or otherwise shaped to provide a -9 supporting stop for the sealing portions 98 of the burst disc. The sealing portions are scored to bend or break from the burst disc and their movement away from the vents 82 is limited by the inner edge of the retainer. The sealing portions contact the sloped surface of the retainer and are stopped before they break free of the burst disc. This also further ensures that a predetermined throttle is formed by the burst disc to throttle the venting of the propellant gases to achieve a complete and controlled propellant burn. A complete and controlled burn of the propellant is needed to achieve predictable muzzle velocities. [0033] Figures 10a, b show an alternative to the rupturable burner in Figures 8a-d. A casing 100 and jacket 101 fit together to enclose a burst ring 102. The casing and jacket include vents 103 and 104 respectively which have corresponding seal portions 105 on the burst ring. An indent at the foot of the casing creates the enclosure for the burst ring. In this example the ring is simply a band of a suitably composed metal or non-metallic substance. External pressure caused by ignition gases from leading projectiles in the stack is resisted by the seal portions. Internal pressure arising from ignition of propellant within the casing causes the seal portions to rapture outwards, releasing gas into the barrel to propel the projectile. [0034] Figures 1 la, b show a variation on the burner in Figures 10a, b. In this example the burst ring 112 has a pair of flanges 113 which clamp the ring in place between the casing 110 and the jacket 111. These flanges assist the sealing action of the burst ring inside the casing. [0035] Figures 12a, b show a further rupturable burner system. A casing 120 is surrounded by burst jacket or sleeve 121. A disc 122 closes the casing once propellant has been loaded. The casing includes vents 123 which are sealed by respective portions 124 in the jacket. Figure 12b shows typical scoring patterns on the jacket, arranged in correspondence with the vents 123. External pressure caused by ignition gases from leading projectiles in the stack is resisted by the jacket. Internal pressure arising from ignition of propellant within the casing causes the jacket to rupture outwards in the vicinity of the vents, releasing gas into the barrel to propel the projectile. [0036] Figures 13 a, b show a further rupturable burner system in which the casing 130 itself includes rupture portions 131. A disc 132 closes the casing once propellant has been loaded. Each portion 131 is formed as an approximately U shaped area surrounded by a channel 133 or otherwise asymmetrically weakened structure in the casing. The detailed structure of the rupture portions is intended to break more readily under outward rather than inward pressure, - 10 as an inherent property of the geometry of the chamber. Multiple rupture portions are formed around a circumference in the casing. The seal which is effectively formed by the casing itself is broken when pressure inside the casing rises after ignition of the propellant, but remains unbroken by relatively lower pressures outside the casing caused by ignition of the other propellant in the barrel. Figure 13c shows the structure and rupture action of the casing in more detail. [0037] Figures 14a, b show alternative scoring patterns for the casing in Figures 13a, b. In Figure 14a a pattern of grooves 140 have been formed on the outside surface of the casing, in relation to a pattern of cavities 141 on the inside surface. The patterns are symmetrical around the cylindrical axis of the casing in this example. Relatively thin portions of material 142 inside the casing between the grooves lines and cavities are intended to rupture more readily in an outwards direction under pressure of ignition gases inside the burner. The geometry of the score lines and cavities is indicated in see-through view of Figure 14b. [0038] Figures 15a, b show a burner having a disintegrable seal 151 around a propellant charge 152. The seal may take various structures such as a wax coating which is consumable in nature. A range of compositions and thicknesses of material may be suitable. The charge is confined by casing 153 and a retainer disc or ring 154. An aperture 155 in the disc allows combustion gases to escape after ignition of the propellant 152. However, the nature of the seal and the aperture 155 prevent gases produced external to the burner from disrupting the seal and exposing the propellant to unintended ignition. [0039] Figures 16a, b show a burner having a casing 160 containing a propellant charge 162. A closure 163 completes the casing and includes a series of apertures 164. The seal takes the form of a deformable ring 161 covering the apertures 164. A primer is typically located in a chamber above the charge. On ignition of the charge in Figure 16b, the ring 161 is deformed into an annular space 165 formed outside closure 161 by the shape of the casing, allowing the ignition gases to escape through vents 165 in the closure. The casing may be formed separately or integrally with the projectile. [0040] Figure 16a shows an internal sealing system implemented by a deformable annular ring sleeve. The annular ring sleeve is press fitted over the annulus with a generally cylindrical casing with exit vent holes in it. The top and bottom portions of the unit are connected to the projectile via means not shown in this diagram. When the propellant is - 11 ignited by the primer the pressure develops inside the unit to the predetermined pressure at which the annular ring is designed to deform outwards and allow expanding propellant gases to vent through the exit vent holes in the annulus. The supporting walls of the upper portion of the unit are angled and positioned appropriately in order that the annular ring deforms only to a predetermined position and is retained. Propellant gases are redirected downwards by the supported angled surface of the deformed annular ring and are typically directed through a further series of vent ports in the lower portion of the unit before entering the barrel and propelling the projectile from the barrel. Figure 16b shows the unit in used state when the annular ring has been deformed. This embodiment of the invention requires only a few parts with just the annular sleeve and the preferably also the outer cylindrical surface over which the sleeve is fitted requiring specific attention during manufacture. Furthermore, should the sleeve fracture it will be retained within the projectile. Another advantage of the deformable sleeve version of this embodiment is the build up to the predetermined pressure resulting in a better gas pressure release profile. The annular chamber defined in part by the angled supporting wall furthermore results in more even venting of gas from the further series of vent ports. The downward or axial direction of the further series of vent ports doesn't direct gases directly onto the bore walls. The gas pressure release profile can be easily varied by simply changing the annular sleeve, whereby the projectile can be easily modified for use with different propellants or for predetermining a different gas release pressure profile. [0041] Figures 21 and 24 show the annular ring sleeve setup seal of Figures 16a and 16b where either or both the upper and lower portions of the unit are integral with other pieces of the tail assembly of a stackable projectile. This embodiment reduces the number of parts for a projectile even further. [0042] Figure 17a shows an internal sealing system implemented by a 'disc spring washer' shaped moveable member. The moveable member may be a disc spring press fitted onto a 'bolt' shaped member. The disc spring may be press fit onto the bolt or can be held in place by a sandwiching 'nut' or both. The nut may be threaded onto the bolt or press fit. The bolt is then threaded into position such that the disc spring seals against the lower surface of the flange on the upper portion of the unit. In this version of this embodiment operation the primer ignites the propellant through vent ports in the top portion of the 'bolt'. Once the burning propellant has reached the predetermined pressure set by the spring rate of the disc spring, the unit opens and vents. The pressure at which the disc spring opens and vents can be - 12 further preset be backing off or advancing the bolt. The unit can be designed such that the upper surface of the bottom portion of the 'bolt' provides a horizontal support for the disc spring (Figure 17b) such that it may return to its original position after firing. Alternatively the upper surface of the bottom portion of the 'bolt' is slopes downwardly (Figure 17c). The disc spring of the embodiment in Figure 17c may be a bi-stable spring whereby it will invert and stay in inverted position after venting has occurred. [0043] Figures 18a and 18b show a similar unit embodiment to Figure 17 with the difference that the disc spring (e.g. Belleville washer) is replaced with yieldable (i.e. non-spring. regular washer) whereby the yieldable 'washer' shaped moveable member will permanently deform at a predetermined pressure set by the thickness and other properties of the washer in order to deform rather than flex in order to vent. [0044] Figures 19a and 19b show a similar unit embodiment to Figures 17 and 18 with the washer and bolt combined. The 'washer' shaped member and 'bolt' are formed into one piece. This embodiment reduces the number of parts for a projectile even further. [0045] As with the embodiment of Figure 16, the embodiments of Figures 17 to 19 are easy to manufacture and vent gas evenly. The gas pressure release profile of the embodiments of Figs. 17 to 19 can be easily varied by simply changing the 'washer' shaped member, or in the case of the embodiment of Fig. 19, the combined washer and bolt. Whereby the projectile can be easily modified for use with different propellants, or for predetermining a different gas release pressure profile. [0046] Figures 17a, b, c show a burner having a casing 170 containing a propellant charge 172. A closure 173 completes the casing and defines an exit 174 for ignition gases. The seal is a spring loaded or otherwise flexible ring 171 blocking the exit 174. The ring has a generally annular shape made of metal or plastic or other suitable material. In this example the ring has a stable configuration as shown in Figure 17a, with the exit blocked. On ignition of the propellant gases, the ring temporarily adopts an unstable configuration as shown in Figure 17b or 17c, with the exit open. Once ignition has taken place, and the pressure of escaping gas is reduced, the ring returns to the stable configuration. A collar 175 may optionally be included to hold the ring in place. These components may be threaded, press fit or otherwise held in place by suitable means. [0047] Figures 18a, b show a burner alternative to Figures 17a, b, c. A casing 180 and closure - 13 183 form a chamber which holds propellant 182, with an exit 184. In this case the seal is a deformable ring 181 which blocks the exit, optionally held in place by a collar 185. As before the ring has a generally annular shape made of metal or plastic or other suitable material, and preferably formed separately from the other components. On ignition of propellant 182 the ring deforms under gas pressure to a new configuration as shown in Figure 18b, opening the seal. [0048] Figures 19a, b show a variation on the burner in Figures 18a, b. as before casing 190 and closure 193 form a chamber with an exit 194, containing propellant 192. In this case, the seal is a deformable flange 191 formed integral with the closure 193. On ignition of the propellant, the flange deforms into exit 194 allowing the ignition gases to escape. A collar is unnecessary to hold the seal in place. [0049] Figures 20a, b show a burner having a compound casing 200 formed by a generally cylindrical insert 206 surrounded by a shell 207. The insert might be formed from a conventional shell casing while the shell might be formed separately or integrally with the projectile. A closure 203 blocks an otherwise open end of the insert, held in place by a plug 208 containing vents 209. Propellant 202 is contained in the chamber formed by the casing and closure. The insert is deformable in the vicinity of the closure and on ignition of the propellant, as shown in Figure 20b, spreads outwards into an annular space 205 formed by the internal shape of the shell 207. Ignition gases then escape through the vents to fire the projectile fro the barrel. [0050] Figures 20c, d show how a burner based on Figures 20a, b may be incorporated in a stackable projectile. In this example the closure 203 and plug 208 are integral with the, preferably plastic, tailpiece of the projectile. The casing press fits into the tailpiece from above, and propellant can be loaded into the casing before insertion of the primer. The tailpiece is then engaged with the warhead. Figure 20b is a cross section through the projectile showing the burner sealed with propellant and then after the propellant has been ignited. Figure 20d has corresponding end views of the tailpiece, showing a change in shape of the vents caused by deformation. [0051] Figures 21a, b show a casing 210 and closure 213 containing propellant 212. The closure is formed by a burst disc 215 located beneath a panel 211 with vents 219. A retainer disc 216 holds the burst disc and the panel in place within the casing. The casing may be - 14 formed from a conventional shell, for example, with the otherwise open end 217 of the casing being crimped to confine the retainer disc. On ignition of the propellant, a seal formed by the burst disc is opened by deformation to release ignition gases through the vents. Ruptured portions 213 of the burst disc are urged outward and are confined by the internal shape of the retainer disc. The burst disc may be weakened in a central region 214, or using an alternative pattern, to enable and control the rupture. [0052] Figure 22a shows an alternative casing 220 with a simple closure 223, containing propellant 222. The otherwise open end of the casing is crimped to confine the closure which preferably disintegrates on ignition of the propellant. Figure 22b shows a further alternative casing 224 which is simply deformed at the otherwise open end 226 to contain the propellant 225, and does not require a separate closure. Figure 22c shows the open form of these casings after ignition of their respective propellant. Figures 23a, b show how burners formed according to Figures 22a, b respectively may be located in stackable projectiles. [0053] Figure 24 shows how a rupturable burner alternative to Figures 13 a, b, c may be formed. In this example the burner is integral with a tailpiece 245 for the projectile. The casing 240 contains propellant 242 and includes rupture portions 241. Each portion is formed by a relatively thin corner 243 which ruptures under pressure caused by ignition gases. Outside the rupture portions the tailpiece includes vents 244. On ignition of the propellant the rupture portions are opened and deform into the volume available in the vents, but leaving an exit for escape of the gases. As before, ignition gases released by other projectiles in a stack remain outside the casing and do not affect the rupture portions. [0054] Figure 25 shows a further stackable projectile as a non-explosive smaller calibre alternative to the projectile of Figure 1. These projectiles are also intended to be loadable and if necessary unloadable in the field. In this example, the projectile has an integral outer casing 250 which contains propellant 251, an inductor and detector system 252, primer and retaining ring 253 actuated by the detector system, and a sealing valve 254 shown in schematic form. The valve may take a variety of structures based on those shown above. [0055] Figures 26a & b show how the projectile in Figure 25 may be stacked. Projectiles 260 and 261 are leading and trailing projectiles respectively, stacked nose to tail in barrel 262. Inductors 263 outside the barrel interact with inductors in the projectiles to initiate the firing process. A breech plug 264 supports projectile 261 at the base of the stack. The projectiles - 15 generally have a sliding fit within the bore of the barrel, and usually include driving bands, but there is generally enough tolerance within the bore of the barrel for hot, high pressure propellant gas from a leading projectile to circulate past trailing projectiles when the leading projectile is fired. In Figure 26b the gas (shaded) from ignition of propellant in the burner of projectile 260 blows backwards down the barrel past projectile 261. Without sealing, there is a tendency for ignition of the propellant in projectile 261. Conventional forms of sealing such as nose to tail wedging may also be employed.