Bolt
The part that locks the cartridge into the chamber and houses the firing pin.
The bolt is the moving piece that closes the breech behind a chambered round. In a manually-operated rifle (bolt-action), the shooter cycles the bolt by hand. In a semi-auto, the bolt rides in a bolt carrier and is cycled by gas or recoil. Locking lugs on the bolt engage the barrel extension or receiver to contain firing pressure.