Although firearms have changed a great deal since they were first invented, the terms used for their parts have changed very little. All modern firearms have three basic groups of parts.
Action: The action is the heart of the firearm—the moving parts that load and fire the ammunition and eject the shells or cartridges. Several types of actions are used in modern firearms. Muzzleloaders have locks instead of actions.
Stock: The stock is the part of the firearm that is held by the shooter. The stock also serves as the platform for supporting the action and the barrel.
Barrel: The barrel is the metal tube that the projectile travels through as a result of pressure from burning gunpowder, compressed air, or other like means. The barrel also guides the projectile in the intended direction.