A breech mechanism for a cannon has a slidable breech block on which is carried a slider that in turn carries a primer-cartridge magazine alignable with a chamber on the block. The slider is automatically driven in the opposite direction as the block when the latter is slid aside to remove fired ammunition and automatically ejects the spent primer cartridge in the breech block during this operation and loads a fresh one from the magazine. A hand crank allows the slider alone to be displaced back and forth between its end positions independent of the motion of the breech block when a primer cartridge misfires.