Bolt
The cylindrical part that closes the chamber, locks against firing pressure, and houses the firing pin and extractor in a bolt-action rifle.
The bolt is the heart of a bolt action. Its lugs rotate into recesses to lock the chamber closed against the thousands of pounds of pressure a shot generates, and its face sets headspace against the cartridge. Inside it carries the spring-driven firing pin, and on its rim the extractor that pulls the fired case back out.
Because the bolt contains and aligns the cartridge at the moment of firing, its fit matters to accuracy. Tight, square lockup and consistent bolt-face contact are part of why bolt guns dominate precision shooting.