Caseless ammunition is firearm ammunition that aims to eliminate the metal case that typically holds the primer, or igniter, and the explosive charge ("gunpowder") that propels the bullet.

In typical caseless ammunition designs, the powder, primer, and bullet are held together with a binding agent. Other possible caseless systems might involve loading only projectiles and using a chemical or other explosive agent that could be ignited electrically by the weapon, rather than mechanically.

Caseless ammunition would have a variety of advantages. Removing the need to eject an empty metal casing would allow a firearm to be simpler and better sealed against dirt and moisture. The hazard of the hot casings themselves would be eliminated, allowing firearms to be used by right- and left-handed users without modification.

The elimination of casings for heavier weapons is particularly attractive for military aircraft. Empty casings are problematic for moving aircraft: ejecting them creates the risk of foreign-object damage, and making provision to retain them requires extra space. The GAU-7 cannon was an attempt to implement this concept.

Caseless ammunition would be beneficial to infantry forces owing to its lighter weight. Its reduced weight would enable the entire logistics supply system, including the end-user, to carry much more ammunition. Increased supply would increase the practicality of weapons with a high rate of fire and of longer missions, while requiring fewer resources be devoted to supply chains. The lack of casing also would provide a tactical advantage to any soldier in a hostile environment. A soldier could carry the same amount of ammunition as with regular cased rounds, but at a reduced weight to increase user endurance and mobility. There would no longer be the danger that a soldier would be spotted due to casings being ejected alongside him. Having no casing would also provide less evidence of a soldier ever having been present.

The lack of casings also makes such ammunition, at least in theory, cheaper to manufacture, since it would consume less material. Reduced cost would further enable greater use.

Caseless ammunition is still experimental, and has not been perfected. Binding agents, used to coalesce the powder and priming charge, can foul weapons. Rounds of this kind of ammunition do not hold up in wet climates or under the stress of combat conditions and tend to stress poorly in magazines and high rate of fire weapons, often breaking or otherwise disintegrating.

An additional problem for fast automatic weapons is heat build-up. An advantage of conventional metal cartridges is that the casing absorbs a large portion of the waste heat of firing. Ejecting the hot, empty casing removes that heat from the weapon, avoiding problems with "cooking off" (premature firing of rounds due to a weapon's extreme heat) of unfired ammunition. With caseless rounds, other means of reducing waste heat are necessary, especially in automatic fire.

Caseless ammunition also has problems with recoil. As the ejection of empty casings allows the exit of gas built up during firing, it reduces recoil as well as ballistic velocity. A closed system using caseless ammunition therefore has significantly increased recoil, albeit with higher muzzle velocity.

The Heckler & Koch G11 assault rifle is perhaps one of the better-known weapons using ammunition of this type, though that rifle never entered full production. It went through a number of prototype stages as well as field testing before being put aside in favour of more conventional firearms.

Ammunition