breech·block /ˈbriʧˌblɑk/
  閂體尾栓,螺體
  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 It is withdrawn for the insertion of a cartridge, and closed again before the gun is fired.
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  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       n : a metal block in breech-loading firearms that is withdrawn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]