Definitions for "Breechblock"
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 It is withdrawn for the insertion of a cartridge, and closed again before the gun is fired.
That part of the action which, being locked into position, supports the cartridge in the chamber of the gun, so that the case may form an effective gas seal when the weapon is discharged. Although a bolt is a breechblock it is not normally so called.
a metal block in breech-loading firearms that is withdrawn to insert a cartridge and replaced to close the breech before firing