Pressure Sustaining Cartridges 6o2f4h

This article was first published in the IAA Journal Issue 406 March - April 1999 International Ammunition Association Inc.

PRESSURE - SUSTAINING CARTRIDGES • an Overview of the Design and Construction of Pressure-Sustaining Small Arms Cartridges by David Tucker INTRODUCTION Silent firearms 1 can be divided in two groups based on the method used to ‘silence’ the firearm: 1. the gun method - where a device, a ‘silencer’, is attached to the barrel and used to reduce the muzzle blast by controlling the flow of propellant gases from the gun. 2. the ammunition method - where the peculiar construction of the ammunition retains the propellant gases within the cartridge case thereby reducing or eliminating the muzzle blast. Cartridges for the ammunition method have been variously referred to as captive-piston, pressure-sustaining, and closed-system propellantexpansion. All have the same basic operating principal of retaining the propellant gases within the cartridge case and differ only in the detail design of the cartridge. HISTORY Pressure-sustaining firearms and cartridges have a long, but thin history with Bissell 2 in 1900 proposing the use of an outwardly conventional gun and cartridge whereby the sudden expansion of the gases at the muzzle of the gun is prevented, thereby insuring a noiseless discharge. A cartridge [ Figure 1 ] contained a projectile in the cartridge neck and a propellant charge to the rear beneath a piston.. A non-compressible medium, which may be formed from a liquid,

such as water, interposed between the projectile and piston. When the cartridge was fired the piston was driven forward by the expanding propellant gases, the movement being transferred to the projectile by the liquid-transmitter. Bissell did not propose to seal the bore, but rather arranged for a slow, controlled release of the propellant gases. Teat 9 rather than piston 3 was the gas seal where the teat was made sufficiently long to project when the piston is in its front position into the barrel and is made of such external diameter as to closely, but not tightly, fit the bore of the barrel or the ed end of the shell 6, and thereby permit a slow flow of gases through the barrel. Although it is not disclosed in the patent, Bissell may have permitted a slow, controlled release of the propellant gases from the cartridge to ease extraction of the fired cartridge case from the gun chamber and to clear the barrel of the liquidtransmitter. The proposed cartridge was to be of such a size as to contain in addition to the piston and explosive material sufficient liquid to at least fill barrel 1, so that the projectile will be subject constantly to a propulsive force until it es from the barrel. A second shot was therefore not a possibility until the barrel had been cleared of the non-compressible medium. Hutfless 3 in 1901 disclosing a captive-piston design for a silent firearm where a piston, used to push

the projectile, was captured and retained within the barrel by a constriction at the muzzle. The propellant gases did not exit the barrel through the muzzle; instead the gases were bled off through holes in the side of the barrel to a silencing device and then vented to the atmosphere. Hutfless did not describe a pressure-sustaining cartridge and only disclosed that … special shells are used to load this weapon. They consist of two parts … loosely connected to one another. No information is provided as to how the shells [ piston and projectile ] are assembled together as a unit [ cartridge ]. While the Hutfless weapon system may have provided a silent discharge, the design was not very practical as the piston, remaining in the barrel, is removed by driving it out with a rammer. A rapid second shot was not a possibility. Holmes 4 in 1922 disclosed a similar silent firearm where a piston was used to propel a projectile, the piston being retained within the barrel by a constriction at the muzzle. As in Hutfless, the propellant gases were bled off from the barrel and vented to the atmosphere through a silencer. The design would have provided a silent discharge but again was not very practical, indeed it is doubtful that some parts of the Holmes design would have functioned as described in the patent.

FIGURE 1 – from United States patent 692,819 Bissell Gun and cartridge for silent discharge where the cartridge contained a liquid-transmitter between projectile and piston. The cartridge contained sufficient liquid-transmitter to fill the barrel so that the projectile will be subject constantly to a propulsive force until it es from the barrel.

In a second patent 5 [ Figure 2 ] Holmes described two pressuresustaining cartridges that are similar in basic design but differ in how the propellant gases were contained within the cartridge [ or vented to the atmosphere ]. For the first variant, the cartridge case 1 was not strong enough to contain the gas pressure and the case relied upon the barrel 15 as in a conventional firearm. Projectile 8 was attached to piston 6 by a rivet 9 and, when the piston struck the buffer 12; the rivet pulled through the piston to create a small hole that vented the propellant gases to the atmosphere. The second variant proposed a cartridge case of sufficient strength to contain the gas pressure by itself and the piston did not perforate. Propellant

gases were retained within the cartridge case for a silent discharge. Aside from these differences, both of the cartridges had the same design features where a hollow piston formed the powder chamber and was used to push the projectile. A groove 7 located the piston and had to be over-ridden by the piston to provide ‘shot start’ resistance for consistent propellant ignition and burning. A buffer 12 located by a groove 14 was used to cushion the piston at the end of the acceleration stroke and a diaphragm 13a between the buffer and the case13 waterproofed the cartridge. For a small arms cartridge Holmes disclosed an expanding dart … the forward end is formed with a conical orifice 10 having its base at the forward end of the projectile, which is

split at 11 so that on hitting its target the forward ends open out on the splits thus giving it the disruptive effect of an explosive bullet and enabling a long thin projectile to execute great damage and shocking power. While some aspects of the design of the Holmes gun [ first patent ] may not have been practical, the proposed pressure-sustaining cartridges [ second patent ] contained design features that would re-appear later in practical, pressure-sustaining cartridges.

FIGURE 2 – from United States patent 1,416,428 Holmes In Fig 2 the propellant gases are vented to the atmosphere when rivet 9 was pulled through and perforated piston 6. Fig 3 shows a pressure-sustaining cartridge where the propellant gases were retained within the cartridge case for a silent, flashless and smokeless gun.

WHISPER During the 1950’s the United States Army had established program WHISPER 6 to develop a completely noiseless, flashless, and smokeless small arms round of ammunition for existing conventional firearms. As part of the Whisper program, Frankford Arsenal is known to have developed at least two pressuresustaining cartridges:

1. Cartridge, Calibre .30, XM76 was intended for use in the M1 ‘Garand’ rifle and fired an 85 grain bullet at 800 fps; the normally semi-automatic rifle functioned as a manual repeater. 2. a calibre .38 cartridge for use in a revolver that fired a 125 grain bullet at 400 fps. Nomenclature of this cartridge is not known. Whisper cartridges were for use in existing small arms and therefore had an outwardly conventional appearance

to assist feeding cartridges from magazine to gun chamber. The cartridge case was made of steel, with thick walls for sufficient strength to contain the gas pressure after the cartridge had been fired and could not ‘crush up’ for a correct fit in the gun chamber. To overcome this the case was made in two parts, the sliding fit between the case body 1 and the case head 2 permitted the case to crush up for correct headspace.

FIGURE 3 - From United States patent 4,173,186 Dunham. FIG 1 and FIG 2 show the operation of the cartridge with a mechanical or solid piston that pushes the bullet from the case. FIG 3 describes an alternate hydraulic piston.

Grooves 8 provided a means of locking the two parts of the case together under pressure from the propellant gases after the cartridge was fired. A similar groove 9 locked the primer 23 [ and 13 ] in the case. The bullet 3 was pushed from the case by a long piston 4, the length of the piston being limited by the overall length of a fired round which had to remain short enough to extract and eject from the firearm. Dunham also disclosed the use of a hydraulic piston to replace the mechanical or solid type. A pusher 21

and piston 22 of a fluid, soft wax or other material replaced the mechanical piston. The hydraulic piston was apparently more efficient as experimental firing data shows a significant increase in velocity over the solid piston when both are fired with the same propellant charge and the same weight projectile. The seemingly complicated piston seal was the result of the failure of a neoprene O-ring in a groove on the piston to provide an adequate gas seal. Slow reaction time of the O-ring to gas pressure allowed propellant gases to

escape around the piston before the Oring expanded sufficiently to seal the gap between piston and case. An effective seal was obtained by using gas pressure acting on the base cup 7 to compress the O-ring 6 against the back-up washer 5. Dimensions of the base cup were such that the cup 7 could not seat against the base of the piston 19 and the force generated by the burning propellant was transmitted to the piston through the compressed O-ring. [ In the early 1970’s the US Navy designed a pressure-sustaining weapon

FIGURE 4 - Silent Weapon System - ALPHA 3 cartridge The cartridge case was 4 inches in length and made of steel, the piston of titanium and the buffer-guides of cellular aluminium. The cartridge fired a 450-grain bullet at 1,000 fps.

system 7 and the gas seal was a neoprene O-ring located in a groove on the piston. The lower ballistic performance of the Navy weapon may have made the complex Whisper piston gas seal unnecessary. ] A means of buffering the piston at the end of the acceleration stroke was provided by the interaction of the piston shoulder 14 and the internal case shoulder 15. The piston shoulder 14 deformed to decelerate the piston and to provide a seal between the piston and case. Metal from the piston shoulder 14 was not only displaced inward to space 20, but also rearwards to space 17 on the piston. The rearward displacement of shoulder 14 was to prevent the piston from being ‘pinched off’ at about 25. Even so the piston had a

taper 27 to 28 and a similar taper inside the case to retain the piston in the event that the piston did fail. SILENT WEAPON SYSTEM - ALPHA Trials of the cartridges developed under the WHISPER program led to a request in 1961 from US Special Warfare Forces for the development of a semi-automatic, silent weapon system. US Army Ordnance Corps 8 established project SILENT WEAPON SYSTEM - ALPHA 9 to develop the proposed weapon system with Springfield Armory given the responsibility for the design and development of the gun and Frankford Arsenal the pressure-sustaining

cartridge. Frankford Arsenal was successful in developing an experimental cartridge 10 [ Alpha 3 ] that proved the feasibility of the ammunition system [ Figure 4 ] when fired in a test fixture. While there are common design characteristics with the earlier Whisper cartridges, there are also differences. The most obvious difference was the change in the shape of the cartridge to that of a cylinder with the bullet telescoped inside the cartridge case. Without the need for the cartridge to function through existing firearms, the case design was orientated towards the ballistic need of a long piston stroke within the allowed case length. The combined buffer-seal-guide for the bullet and piston was

made from a form of cellular aluminium that crushed to form the buffer. The initially guided the projectile, was then stripped from the projectile by the bushing and crushed to provide a buffer to decelerate the piston and a seal between piston and bushing at the case mouth. While functionally the same as the Whisper design, the piston seal of the Alpha 3 cartridge used a simplified arrangement where a washer replaced the more complex base cup of the Whisper cartridge. As in the Whisper cartridges, copper pins were used to locate the projectile and piston within the cartridge case and to provide ‘shot start’ resistance for more consistent interior ballistics.

The subject of lethality was also addressed in the design of the Alpha 3 cartridge. Drawing on wound ballistic research carried out at Frankford Arsenal and elsewhere, a projectile with a right-circular cylindrical body of Elkonite with a hemispherical nose of aluminum … it is fired with a minimum spin so as to be just sufficiently stable in flight. The heavy body-light nose design increases the moment of coefficient and decreases the traverse moment of inertia to ensure instability in the target medium. Or, in plain language, the projectile was designed to tumble on impact causing a larger, more destructive wound.

UNDERWATER AMMUNITION AAI Corporation of Cockeysville, Maryland developed this cartridge as part of an underwater weapon system that had the form of a revolver. Firing conventional small arms underwater involved several problems including the functioning of the weapon, limited range and the ‘noise’ that could seriously effect a diver. AAI sought to overcome these problems with a silent weapon that fired a fin stabilised steel dart weighing 155 grains at 700 fps 11. The cartridge [ Figure 5 ] was of cylindrical form for a long piston acceleration stroke and to hold the dart telescoped within the cartridge.

FIGURE 5 - from United States patent 3,476,048 Barr and Critcher Developed by AAI Corporation for an underwater weapon system of their design. Several features were carried over to ammunition for the Tunnel Weapon developed for the United States Army.

A compressed neoprene O-ring gas seal was not used instead grooves and threads on the piston acted as a gas seal and a means of buffering the piston at the end of the acceleration stroke. A close fit of the piston in the case together with grooves 17a provided a piston-case seal as propellant gases leaking the piston were retarded by adiabatic expansion and temperature reduction as the compressed gases entered the grooves and expanded. Threads 17b and 17c on the forward portion of the piston acted as buffer-seals by shearing and deforming when the piston struck the lip 13c to slow the piston and to seal the case and retain the propellant gases. In the second patent, the forward portion of the threads was filled with soft, copper

wire that was deformed along with the threads to decelerate the piston and provide a gas seal. As well as being ed at the front and rear ends, the dart 15 was also ed and guided at the midsection by s 31, 33 and 35 that centred and prevented the slim dart from bending under acceleration forces. Front guides 37, 39 and 41 not only centred and ed the dart but also acted as seals to exclude water from the interior of the case. Water inside the case would have slowed the acceleration stroke of the piston and reduced the velocity of the dart. It is not known if AAI developed this underwater weapon system at the request of a Government Agency or if the work was a Company initiative. Patents for both the ammunition and

the proposed underwater pistols were assigned to AAI and not to a Government Agency. TUNNEL WEAPON In December 1967 the Military Assistance Command, Vietnam identified a need for a low-noise, multiprojectile weapon and ammunition to be used by tunnel exploration personal. The dark, cramped conditions of tunnel searches dictated the need for a handy, compact weapon that was silent and flashless in operation. The Tunnel Weapon 12 or Quiet Special Purpose Revolver [ QSPR ] was a project of the US Army Limited Warfare Laboratory 13, the design and development work carried out by AAI

FIGURE 6 - from United States patent 3,602,143 Critcher Quiet Special Purpose Revolver or Tunnel Weapon cartridge cartridge case length body diameter projectile velocity effective range

steel 1.87” 0.523” 15 steel pellets [ in a sabot ] 730 fps approximately 25 feet

Corporation. The gun was a modified Smith and Wesson .44 Magnum revolver; the barrel shortened to 1 3/8”, the rifling removed and the cylinder bored out to accept the new cartridge. Ammunition for the Tunnel Weapon was similar to that in the patent 14 [ Figure 6 ] and carried over several design features from the Underwater Ammunition. A hollow piston was the propellant chamber and maximised the length of the piston stroke, provided a means of obtaining ‘shot start’ resistance and a mechanical piston seal. Flange 14 abutted shoulder 26 and retained the piston in the rearward position until the cartridge was fired. Interaction of the flange and shoulder provided the necessary ‘shot start’ resistance before the flange sheared and the piston pushed the sabot out of the case. At the end of the acceleration stroke, the piston struck the threads at the mouth of the case and the threads deformed to decelerate the piston. The piston jammed in the threaded portion of the case to form a piston-case seal and retain the propellant gases within the cartridge case. Sound level of the Tunnel Weapon was reported to be 120 decibels at one metre from the muzzle, which is comparable to the silenced .22 caliber pistol. The effective range was short at about 25 feet but not unrealistic given the engagement ranges inside tunnels systems. Ten guns and 992 rounds were sent to Vietnam in 1969 for evaluation and the weapons were actually used during combat operations. Not only were the weapons used as intended for tunnel searches, but the silent and handy nature of the weapon loaned it to unintended uses clearing bunkers and spider holes, and during ambushes. Most s are reported to have expressed enthusiasm for the weapon system as a whole and especially for

use in unintended roles. Criticism related to a lack of lethality, unreliable ammunition traced to the priming system, and a gun problem where the hammer nose ‘mushroomed’ after about 25 rounds had been fired 15. AAI did further development work using a different priming system in an effort to improve the reliability of the ammunition and four variations of the cartridge are known. The separate firing pin 6 was abandoned with the gun hammer

striking the primer directly in the later arrangements. A spring washer was incorporated in to the cartridge between the piston and the case head to hold the piston flange 15 against the shoulder 26 to positively locate the hollow piston for more reliable ignition. Longitudinal slots were also added to the threads 3 at the case mouth. While the purpose of the slotted threads is not known for certain, it is likely that the slots were to allow air,

from inside the cartridge case, to escape around the sabot during the acceleration stroke. Air trapped in the case would have retarded the piston acceleration stroke and reduced velocity. The ACTIV report also recommended that a slug and improved multipellet cartridge be adopted for use in the Tunnel Weapon that will be lethal at 25 feet, when fired into a vital part of the body. There is an unsubstantiated report of a four-segment bullet being used to improve lethality. NAVY 12-GAUGE SHOTGUN AAI also developed silent 12-gauge shotgun ammunition that did not use the captive-piston method of the Tunnel Weapon but a flexible metal membrane or diaphragm. During the 1950’s AAI had developed a method of forming a flexible metal diaphragm that unfolded under propellant gas pressure. How the system works is best understood from the patent drawings 16 [ Figure 7 ]. Gas pressure, generated by the burning propellant, unfolds the steel diaphragm 40 and ‘fires’ the payload from the ‘cartridge case’. The diaphragm system was not at first used for silent ammunition but in various devices where the reliability and power of burning propellant was required without the possibility of explosive damage or chemical contamination [ the patent illustrates an ‘explosive bolt’ used to separate the stages of a rocket ]. In 1967 the United States Navy requested a silent, flashless, smokeless, 12-gauge shotgun round and AAI applied the diaphragm system to ammunition. A steel cartridge case contained a plastic ‘pusher’, corresponding to 34, and was loaded with 12 No. 4 lead buckshot [ diameter

FIGURE 7 - from United States patent 3,119,302 Barr In 1967 AAI Corporation applied the diaphragm system to silent, flashless, smokeless, 12- gauge shotgun ammunition for the United States Navy

0.24” ] that were fired at 450 fps. Reportedly only one lot of 200 rounds of ‘Teleshot’ ammunition was made for testing by the US Navy 17. The ‘Teleshot’ system of a flexible metal diaphragm was later applied by AAI to silent ammunition for the 40mm grenade launcher that used a rifled cartridge case in place of the usual gun barrel. NOTE A private individual has produced silent ammunition for the 12-gauge shotgun using a simplified version of the Alpha 3 - AAI captive piston design 18 . The cartridge shows how ingenuity can overcome the technical problems involved and the level of ballistic performance possible from pressure-sustaining ammunition. A metal cartridge case contained a piston in the form of a simple disk of Nylon a ¼” thick. The piston was also the gas seal as it interacted with a ‘lip’ at the case mouth to retain the propellant gases inside the case. The projectile was a steel dart; body diameter of 0.310” with four bore size plastic fins and weighing 240 grains. Reported velocity was 585 fps using a charge of 6.0 grains of Hercules Bullseye propellant. 1

pneumatic and spring-powered guns and similar devices that are ‘silent’ in operation are ignored, as are the various types of spigot-launch guns. 2 United States patent 692,819 Bissell 11 February 1902 3 Austria patent 5478 of 1901 Hutfless. 4 United States patent 1,416,827 Holmes 23 May, 1922. 5 United States patent 1,416,428 Holmes 23 May, 1922 6 United States patent 4,173,186 Dunham assigned to the US Army. The patent application is dated 7 July,

1960 but was not granted for 19 years until 6 November, 1979. 7 United States patent 3,837,107 Swaim and Widmayer 24 September, 1974 assigned to the US Navy. 8 later US Army Materiel Command 9 Ordnance Weapon Command, Report 613301. 10 Army Munitions Command, Frankford Arsenal Report R-1654. 11 United States patent 3,476,048 Barr and Critcher 4 November, 1967 and 3,585,934 Mueller and Critcher 22 June, 1971 assigned to AAI Corporation. 12 Limited Warfare Laboratory Task No. 02-F-68. 13 later Land Warfare Laboratory 14 United States patent 3,602,143 Critcher assigned to the US Army. 15 Army Concept Team in Vietnam ACTIV Project No.ACG-25/691, Final Report, November 1969. 16 United States patent 3,119,302 Barr 28 January 1964 assigned to Aircraft Armaments Corporation later AAI Corporation. One of a series of related patents for diaphragm ‘cartridges’. 17 The World’s Fighting Shotguns, Thomas F. Swearengen. 18 The Silent Shotgun Shell, Alan Clendinen, Firepower, November 1987.