The Collaborative International Dictionary of English v.0.48:

Breechblock \Breech"block\, 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   is withdrawn for the insertion of a cartridge, and closed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]

WordNet (r) 3.0 (2006):

breechblock
    n 1: a metal block in breech-loading firearms that is withdrawn
         to insert a cartridge and replaced to close the breech
         before firing [syn: breechblock, breech closer]