bolt

A bolt is the part of a repeating, breechloading 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 firing pin and extractor are often integral parts of the bolt. The terms "breechblock" and "bolt" are often used interchangeably or without a clear distinction, though usually, a bolt is a type of breechblock that has a nominally circular cross-section.