In firearms terminology and at law, the firearm frame or receiver is the part of a firearm which integrates other components by providing housing for internal action components such as the hammer, bolt or breechblock, firing pin and extractor, and has threaded interfaces for externally attaching ("receiving") components such as the barrel, stock, trigger mechanism and iron/optical sights.
A disassembled Mauser action showing a partially disassembled receiver and bolt
AR-15 rifles showing their configurations with different upper receivers. The lower receiver is visible at the bottom
In firearms terminology, an action is the functional mechanism of a breech-loading firearm that handles the ammunition cartridges, or the method by which that mechanism works. Actions are technically not present on muzzleloaders, as all those are single-shot firearms with a closed off breech with the powder and projectile manually loaded from the muzzle. Instead, the muzzleloader ignition mechanism is referred to as the lock.
A view of the break-action of a typical double-barreled shotgun, with the action open and the extractor visible. The opening lever and the safety catch are clearly visible.
Smith and Wesson M&P revolver
Barreled action for bolt-action rifle
U.S. M1895 Lee Navy straight-pull rifle