This article relies largely or entirely on a single source. (December 2009)
A bolt is the part of a repeating, breech-loading firearm that blocks the rear opening (breech) of the barrel chamber while the propellant burns, and moves back and forward to facilitate loading/unloading of cartridges from the magazine. The extractor and firing pin are often integral parts of the bolt.
In gas-operated firearms, the bolt itself is housed within the larger bolt carrier group (BCG), which contains additional parts that receives rearward push from the gas piston.
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the bolt is held fixed by its locking lugs during firing, forcing all the expanding gas forward, and is manually withdrawn to chamber another round.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. Once the case is clear of the chamber, the ejector kicks the case out of the firearm. When the bolt moves forward, it strips a cartridge from the magazine and pushes it into the chamber.
The most common locking mechanism on rifles is a rotating bolt, which can be classified as a rigid type of bolt lock. Semi-rigid bolt locks have their locking elements movably mounted on either the bolt, barrel or breech housing. Examples of semi-rigid bolt locks are roller locked bolts, or ball bearings as on Heym SR 30 or Anschütz 1827 Fortner (both straight-pull rifles).
In a closed bolt firearm, the bolt is in its foremost position upon firing. This is opposed to an open bolt firearm where the bolt is held rearward and pulling the trigger releases it to slam forward and fire the cartridge.
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