1) Block 2) Blockage 3) Breech closer 4) Closure 5) Firearm component 6) Occlusion 7) Stop 8) Stoppage 
A breechblock (or `breech block`) is the part of the firearm action that closes the breech of a weapon (whether small arms or artillery) at the moment of firing. Most modern small firearms use a bolt. Some variations of breechblocks include: A less common type of breech is the split-breech design known as the `nutcracker`. This type of breech ...
• (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 It is withdrawn for the insertion of a cartridge, and closed again before the gun is fired.
Breech'block noun 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 It is withdrawn for the insertion of a cartridge, and closed again before the gun is fired. 
[n] - a metal block in breech-loading firearms that is withdrawn to insert a cartridge and replaced to close the breech before firing
breech closer noun a metal block in breech-loading firearms that is withdrawn to insert a cartridge and replaced to close the breech before firing
a movable piece of metal for closing the breech in certain firearms.