Source : Webster's Revised Unabridged Dictionary (1913)

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   is withdrawn for the insertion of a cartridge, and closed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.

Source : WordNet®

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     n : a metal block in breech-loading firearms that is withdrawn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