The gas-operated system for implementing automatic reloading of a firearm is one of five such systems, the others being recoil-operated, Gatling, chain, and blowback. Recoil operation is one of the firearm actions used in automatic firearms. ... Gatling gun illustrated in an 1885 encyclopedia in Swedish The Gatling gun was the first successful machine gun. ... The M242 25mm Chain gun A chain gun is a type of machine gun or autocannon that uses an external source of power, rather than recoil, to cycle the weapon. ... Blowback is one particular system in which an automatic or semi-automatic firearm may operate. ...

In a gas-operated system some of the expanding gases from the round being fired are routed from a port just behind the muzzle (where the pressure is lower) in front of a piston, typically lying over the barrel. The piston is forced backwards by the gas, and is connected to the breech of the gun, which is unlocked by the backwards motion induced by the piston when the round leaves the barrel. This system is useful for machine guns and assault rifles, and permits use of a long barrel and makes efficient use of the propellant.

Usually, the speed at which the weapon fires is determined by a regulator, which controls the gas flow to the piston. The greater the gas flow, the faster the weapon will fire. This also means the faster the shot is fired, the more warping and jamming caused by overheating.

Weapons using this type of automatic reloading include the HK G36, AK-47 and derivatives. The M-16 uses direct impingement, which is a version of gas operation without a piston. H&K G36 The Heckler und Koch Gewehr Model 36 (company designation G36, Bundeswehr designation ) is a modular weapon system designed in West Germany in the 1980s and 1990s. ... Avtomat Kalashnikova model 1947 g. ... ... A gas-operated firearm that uses direct impingement is a firearm where gas from a fired catridge is routed directly to bolt carrier or slide assembly to cycle the action in the firearm. ...