(n) a metal block in breech-loading firearms that is withdrawn to insert a cartridge and replaced to close the breech before firing, Syn. breech closer
The Collaborative International Dictionary of English (GCIDE) v.0.53 Collaborative International Dictionary (GCIDE)
n. The movable piece which closes the breech of a breech-loading firearm, and resists the backward force of the discharge. It is withdrawn for the insertion of a cartridge, and closed again before the gun is fired. [ 1913 Webster ]