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ARMAG

Search matches names, aliases and the text of every page. An arm found by its service designation is found by its alias, not its title.

Gas operation

An action driven by propellant gas tapped from the barrel through a port, acting either on a piston or directly on the bolt carrier.

Also called Gas system

Most self-loading rifles work this way. A small hole in the barrel wall, some distance ahead of the chamber, lets a fraction of the propellant gas out while the bullet is still travelling. That gas does work on something, and the something eventually unlocks and cycles the breech.

The three arrangements differ in what the gas pushes. A long-stroke piston is fixed to the bolt carrier and travels the whole distance with it — the Kalashnikov, the M1 Garand. A short-stroke piston travels a few millimetres, strikes the carrier and stops — the M1 carbine, the SCAR, the SVD. Direct impingement dispenses with the piston: gas goes down a tube into the carrier itself, which acts as its own cylinder, as in the M16.

Gas operation needs a barrel port and enough pressure to tap. A weapon firing low-pressure ammunition may not cycle at all, which is why gas guns are fussier about their loads than blowback ones.