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 rotating breechblock, better known as a bolt, first used in manually operated bolt action weapons, but later operated by the power of the cartridge in automatic weapons. Some variations of non-rotating breechblocks include:
- Blowback breechblock - in which the breechblock is closed by a powerful spring. A variation of this is the "Delayed blowback" found in many weapons from Heckler & Koch
- Tilting breechblock - both self-actuating (AG-42 Ljungman is one example) and manually operated by a lever (Peabody action) or other means (Krag-Petersson)
- Falling block (Sharps rifle)
- Floating breech
- Rolling block (Remington)
- Hinged breechblock (Snider-Enfield, Springfield rifle)
- Screwed breechblock (Ferguson rifle)
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 horizontal or vertical sliding block.