Breechblock

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 consists of two counter-rotating sprockets, with a temporary breech being formed where they touch. Relatively few guns have used this design. The prototype Fokker-Leimberger multiple-barreled machine gun used this design, but it had numerous problems with ruptured cases. Another "Fokker Split Breech Rotary Machine Gun, ca. 1930" was donated to Kentucky Military Treasures; according to the museum record it "proved unsuccessful because of its inability to seal breech cylinders". A couple of 1920s US patents by other inventors also proposed to use this principle. The British also experimented with the design in the 1950s for aircraft guns, without success. It has only been used successfully in low-pressure applications, such as the Mk 18 Mod 0 grenade launcher.

In artillery the forces are much greater, but similar methods are used. The Welin breech block uses an interrupted screw and is used on weapons with calibres from about 4 inches up to 16 inches or more. Other systems use a horizontal or vertical sliding block, in which a solid block is slid across the open breech from the side or bottom to seal the opening.

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