Basic Parts of a Firearm 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, fire, and eject the shells or cartridges. Several types of actions are used in modern firearms. Muzzleloaders have locks instead of actions. Stock: The stock serves as the handle of the firearm. It can be composed of one or two pieces and is usually made of wood or a synthetic material. Barrel: The barrel is the metal tube that the projectile travels through (bullets travel through the barrels of rifles and handguns; shot travels through the barrel of shotguns).