Caseless ammunition is an attempt to reduce the weight and cost of ammunition by dispensing with the case, which is typically precision made of brass or steel, as well as to simplify the operation of repeating firearms by eliminating the need to extract and eject the empty case after firing. Its acceptance has been hampered by problems with heat sensitivity, sealing, and fragility. Its use to date has been limited to prototypes and low-powered firearms.
During World War II, Germany began an intensive program to research and develop a practical caseless ammunition for military use, which was driven by the rising scarcity of metals, especially copper used to make cartridge cases. The Germans had some success, but not sufficient to produce a caseless cartridge system during the war. Japan successfully developed the Ho-301 40mm autocannon during the war for mounting on aircraft. It saw active, though relatively limited, use in the defence of the Japanese home islands during the waning months of the war.
Modern caseless ammunition
Modern caseless ammunition consists of a solid mass of propellant, originally nitrocellulose, cast into shape to form the body of the cartridge, with cavities to accept the bullet and a primer, preferably combustible, which are glued into place. The completed cartridge might also contain a booster charge of powdered propellant to help ignite the body and provide initial thrust to the bullet.
Many caseless cartridges are also telescoped, with the bulk of the bullet held within the body of the cartridge, to cut down on cartridge length. A shorter cartridge cuts down on the distance the firearm's action must reciprocate to load a new round, which allows for higher cyclic rates and greater probability of multiple hits on a target at long range. Lack of a case also reduces the weight of the cartridge substantially, especially in small bore rifles. For example, the caseless ammunition designed by Austrian inventor Hubert Usel for the Voere VEC-91 weighs about one third as much as regular ammunition for the same caliber.
While it seems a simple operation to replace the case with a piece of solid propellant, the cartridge case provides more than just a way to keep the cartridge components together, and these other functions must be replaced if the case is to be replaced. Caseless ammunition is not without its drawbacks, and it is these drawbacks that have kept modern caseless ammunition from achieving wider success.
The first major problem, of special concern in military applications, which often involve sustained firing, is the heat sensitivity of the ammunition. Nitrocellulose, the primary component of modern gunpowder, ignites at a relatively low temperature of around 170 °C (338 °F). One of the functions of the metallic cartridge case is as a heat sink; when extracted after firing, every metallic case carries away a significant amount of the heat from the combustion of the propellant, slowing the rate at which heat builds up in the chamber. The thermal insulation provided by the case also works the other way around, shielding the propellant from built-up heat in the chamber walls.
Without a case to provide these functions, caseless rounds using nitrocellulose will begin to cook off, firing from the residual chamber heat, much sooner than cased cartridges do. Cooking off can be avoided by designing the weapon to fire from an open bolt, but this greatly impacts accuracy, and thus is only suitable for machine guns and submachine guns.
The normal solution to the problem of heat is to increase the heat resistance by switching to a propellant with a higher ignition temperature, typically a non-crystalline explosive carefully formulated to provide an appropriate rate of combustion. Heckler & Koch, in concert with Dynamit Nobel, managed such a task by producing relatively heat-resistant caseless ammunition.
Another important function provided by the cartridge is to seal the rear of the chamber. During firing of a cased cartridge, the pressure in the chamber expands the metallic case which obturates the chamber. This prevents gas exiting from the rear of the chamber, and it has also been experimentally shown to provide a significant amount of support to the bolt. Without the case to provide this seal, the firearm design must account for this and provide a means of sealing the rear of the chamber. This problem was also encountered with the Dreyse needle gun; the French Chassepot solved the leaking-breech problem with the addition of a rubber seal to the bolt.
Telescoping caseless rounds must also deal with the issue of blocking the bore, as the bullet is surrounded by propellant. The booster charge is used to address this issue, providing an initial burst of pressure to force the bullet out of the cartridge body and into the barrel before the body combusts.
Caseless rounds are limited by the fact that the cartridge body is primarily a propellant, and structural properties are secondary to the combustion properties. The primary issue is one of extraction. While caseless ammunition eliminates the need to extract a fired case, unfired cases must be extracted in case of a misfire, or to unload the firearm. With metallic cases, this ability is provided by a rim or extractor groove machined into the rear of the case. Even in completely plastic bodied cartridges, such as the Activ brand shotgun shells, a thin metal ring is molded into the rim to provide support for the extractor.
Modern caseless firearms
One of the first caseless firearm and ammunition systems produced was actually made by Daisy, the airgun maker, in 1968. The Daisy V/L Rifle used a .22 caliber (5.5 mm) low-powered caseless round with no primer. The rifle was basically a spring-piston air rifle, but when used with the V/L ammunition the energy from the compression of the piston heated the air behind the caseless cartridge enough to ignite the propellant, and this generated the bulk of the energy of firing. The Daisy V/L Rifle system was discontinued in 1969 after the ATF ruled that it was not an airgun, but a firearm, which Daisy was not licensed to produce.
Some assault rifles have used caseless ammunition. One of the better-known weapons of this type is the G11 made by Heckler & Koch. Although the rifle never entered full production due to issues with chamber sealing and heat, it went through a number of prototype stages as well as field testing before being put aside in favour of a more conventional firearm, the G36, which was also cheaper.
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- See main article, Chassepot, for references
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- Starry, Donn A., General. Mounted Combat in Vietnam. Department of the Army, Washington D.C., 1978.