A firearm action is the type of mechanism or system used in a rifle, pistol, machine guns, and shotguns to reload a new round in the chamber and eject the empty case of the cartridge that has been fired. There are six types of firearm action used today: bolt-action, break-action, pump-action, blowback, recoil, and gas-operated systems. The first three mechanisms are manually-operated and the remainder three are automatic systems.
Bolt action: It is a manually-operated mechanism, in which a new cartridge is reloaded into the chamber by opening and closing the bolt through a handle that is attached it at a 90º angle. When this handle is pulled back, the empty case is ejected; when it is pushed forward a new cartridge is loaded into the chamber from a fixed magazine. Famous bolt-action weapons are the Lee-Enfield, Springfield M1903, and Mauser Gewehr 98.
Break action: Used in shotguns, it is a manual way of reloading, in which the breech is exposed by breaking open the hinged barrel at the base.
Pump action: Also used in shotguns, it is a system in which the firearm is reloaded by manually sliding back and forth a cylindrical fore handgrip.
Blowback: Mostly used in submachine guns, it is an automatic reloading system that uses the energy created by the cartridge ignition to open and close the bolt.
Recoil: It is a system usually used in pistols and machine guns, employing the energy of the moving pieces recoil to eject the empty case and reload a new cartridge.
Gas: It is a gas-operated system mostly used in modern assault rifles. The rifle bolt carrier is pushed back by the great pressure exerted by gases created by the ammunition ignition. These gases are sucked up into a cylinder, moving back the bolt by direct impingement or through a rod.