The Collaborative International Dictionary of English v.0.48:

Breech action \Breech action\
   The breech mechanism in breech-loading small arms and certain
   special guns, as automatic and machine guns; -- used
   frequently in referring to the method by which the movable
   barrels of breech-loading shotguns are locked, unlocked, or
   rotated to loading position.
   [Webster 1913 Suppl.]