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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.

Open bolt

A firing cycle in which the bolt is held to the rear between shots and runs forward to chamber and fire in one motion.

Also called Advanced primer ignition

Pull the trigger and a heavy bolt, already held back, runs forward under spring pressure. It strips a round from the magazine, chambers it, and a fixed firing pin machined into the bolt face fires it as the bolt closes — sometimes fractionally before, which is why the arrangement is also called advanced primer ignition.

The advantage is cooling: the chamber is open to the air between bursts, so a round cannot cook off in a hot barrel. The disadvantage is accuracy. A quarter of a kilogram of steel moving forward at the moment of firing pulls the muzzle down and forward, and the shot has gone before the weapon settles.

Nearly every simple submachine gun on this site — the Uzi, the MP 40, the Sterling, the MAC-10 — fires this way. The MP5 does not, and that is most of the reason police forces bought it.