A breechblock is the part of the firearm action that closes the breech of a breech loading weapon before or at the moment of firing. It seals the breech and contains the pressure generated by the ignited propellant. Retracting the breechblock allows the chamber to be loaded with a cartridge.
A typical break-action, double-barreled shotgun
A TOZ-17 cadet rifle chambered for .22 long rifle which has been disassembled. The bolt is locked by the bolt handle being dropped into a notch in the receiver.
A Mauser 98 bolt showing two locking lugs just behind the face of the bolt. There is a third "emergency" lug in front of the bolt handle, in case the primary ones fail under pressure. In some designs the bolt handle itself may serve as the emergency "lug".
AR-15 bolt carriers showing multiple locking lugs on the bolts
In firearms terminology, an action is the functional mechanism of a breech-loading firearm that handles the ammunition cartridges, or the method by which that mechanism works. Actions are technically not present on muzzleloaders, as all those are single-shot firearms with a closed off breech with the powder and projectile manually loaded from the muzzle. Instead, the muzzleloader ignition mechanism is referred to as the lock.
A view of the break-action of a typical double-barreled shotgun, with the action open and the extractor visible. The opening lever and the safety catch are clearly visible.
Smith and Wesson M&P revolver
Barreled action for bolt-action rifle
U.S. M1895 Lee Navy straight-pull rifle
