In manually operated firearms, such as bolt-action, lever-action, and pump-action rifles and shotguns, the bolt is held fixed by its locking lugs during firing, forcing all the expanding gas forward, and is manually withdrawn to chamber another round.
In an automatic or semi-automatic firearm, the bolt cycles back and forward between each shot, propelled by recoil or expanding gas (back) or the recoil spring (forward). When it moves back, the extractor pulls the spent casing from the chamber. When it moves forward, it strips a cartridge from the magazine and pushes it into the chamber. Once the case is clear of the chamber, the ejector kicks the case out of the weapon. The extractor and firing pin are often integral parts of the bolt. The slide of a semi-automatic pistol is a form of bolt.
AR-15 bolt carriers
AK-74 bolt and firing pin.
K31 bolt disassembled
Automatic rifle bolt (top) off a SIG SG 510 battle rifle
Slide locked back on a Desert Eagle pistol, showing the gas operated rotating bolt mechanism
- Full Auto describes the function of the bolt in detail