breechblock(n = noun.artifact) breech closer - a metal block in breech-loading firearms that is withdrawn to insert a cartridge and replaced to close the breech before firing;
is a kind of block, blockage, closure, occlusion, stop, stoppage
is a part of breech, rear of barrel, rear of tube

   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 It is withdrawn for the insertion of a cartridge, and closed again before the gun is fired.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]