The mechanical part of a firearm that blocks the rear of the chamber while the projectile is discharged, forcing all the expanding gas forward. In a bolt action firearm, the bolt is operated manually. In a semi‐automatic firearm, the bolt moves forward or backward, under pressure from the expanding gas or the recoil spring. When it moves forward it pushes a cartridge from the magazine into the chamber. When it moves backward, the extractor pulls the spent case from the chamber. Once the case is clear the ejector propels the case out of the weapon. The firing pin, extractor, and sometimes the ejector are usually integral parts of the bolt. See also action; breech block.