Chamber
The end of the bore where the cartridge sits before firing. Cut to specific cartridge dimensions. A .308 chamber will not accept a .300 PRC cartridge.
The chamber’s reamer dimensions affect everything downstream. SAAMI-spec chambers are looser to accept a wider tolerance of factory ammo. Match-grade chambers (Wylde, Bartlein-cut customs) run tighter for handloads and gain a small accuracy advantage but with less feeding margin.
Headspace, the precise distance from the bolt face to a reference point in the chamber, is measured during a build and verified with go/no-go gauges.