The bolt carrier group (BCG), most often found in gas-operated weapons, performs the cyclic functions of the weapon, detailed below.
- Chambering - The bolt inserts the round into the chamber. It is at this point that the extractor catches onto the rim of the cartridge.
- Locking - The bolt locks into place in the barrel extension (usually by rotating; a cam pin follows a track milled into the bolt carrier and rotates the bolt slightly).
- Firing - The sear releases the hammer, which strikes the firing pin (located within the bolt carrier group). This fires the cartridge, which, once the bullet passes the gas port, sends the gas from the fired cartridge back onto either an actuator rod (in short-stroke and long-stroke piston systems), into a chamber located within the bolt carrier (in expanding gas piston systems), or directly onto the bolt face itself (in direct impingement systems).
- Unlocking - The bolt unlocks via the bolt carrier being pushed back, which rotates the bolt.
- Extraction - The bolt, upon unlocking, pulls the cartridge from the chamber.
- Ejection - The bolt contains a spring-loaded ejector. During the bolt carrier group's rearward cycle, the ejector ejects the spent casing via the ejection port.
- Cocking - As the BCG cycles all the way back, it cocks the hammer, allowing the weapon to be fired again.
The BCG is then sent forward, feeding another cartridge and repeating the process. The slide on a semi-automatic pistol performs these same functions, albeit with slightly different parts and a different operating system altogether (as most pistols are not gas-operated).