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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.

Blowback

An action in which nothing locks the breech: the bolt is held closed only by its own mass and the recoil spring.

Also called Simple blowback, Unlocked breech

The simplest way to make a self-loading firearm work, and the reason almost every submachine gun and .22 rifle uses it. There is no locking mechanism at all. When the cartridge fires, the case pushes back on the bolt face and the bolt starts moving immediately; what keeps the breech effectively shut for the fraction of a millisecond that matters is that the bolt is heavy and the spring behind it is stiff.

That works up to a point, and the point is chamber pressure. A 9×19mm round at 235 MPa can be held closed by a bolt of reasonable weight. A rifle cartridge at 400 MPa cannot — the bolt would have to weigh several kilograms. Everything above pistol pressure therefore needs either a lock or a delay.