The Uzi submachine gun is an example of a modern fully automatic firearm.
The operating action of modern automatic firearms can be divided into two broad classes: semiautomatic and fully automatic. Full-auto weapons include machine guns, submachine guns, assault rifles or PDW (personal defense weapons) such as the Uzi. Semiautomatic categories encompass a wide variety of civilian handguns, rifles and shotguns. Semiautomatic groups can be further broken down into single-action or double-action varieties. These classifications refer to the mechanical manner in which the firearm is cocked, fired and reloaded for the next shot.
Fully automatic weapons are legally available only to the military and law enforcement in most countries. They are capable of delivering hundreds of rounds per minute to a target. In a fully automatic weapon, the trigger is pulled once and held. The firearm will fire a round, eject the spent cartridge, chamber the next round, cock the hammer and fire the next round. This cycle of automatic firing, ejecting and chambering continues at a rapid pace as long as the trigger is depressed. The action is powered either by the recoil energy from each shot or by gases from the fired round, which drive a piston that operates the bolt, extractor, cocking and firing mechanisms. Ammunition is fed into an automatic weapon from a box or drum magazine or from a belt threaded into the receiver. The British Maxim gun, invented in 1884, is considered to be the first fully operational automatic weapon.
Semiautomatic weapons — often known as auto-loaders — require the operator to pull the trigger to fire each round. However, the automatic component of semiauto action ejects the spent cartridge and chambers the next round, eliminating manual cycling or pump-action between shots. The action is powered by the recoil energy from each shot or by expanding gases from the fired round, which drive a piston that operates the bolt, extractor and cocking mechanism. Most semiautomatic firearms will fire as fast as you can pull the trigger. Weapons with semiautomatic operation include rifles, shotguns and handguns.
In a single-action semiautomatic firearm, the hammer is cocked for the first round by the loading process of pulling the slide or bolt back to chamber the round. Firing the first and each subsequent round requires only pulling the trigger. After each shot the hammer is automatically recocked by the action, and the operator is required only to pull the trigger to fire the weapon again. Because hammer cocking is performed by recoil or gas operation of the gun, single-action firearms have a much lighter trigger pull than double-action firearms.
Double-action semiautomatics eject the cartridge after each shot and rechamber the next cartridge. However, the hammer or striker remains in the uncocked position. Cocking the hammer or striker and releasing it for each shot is part of the mechanical sequence of each trigger pull. Double-action operation usually results in a longer, heavier trigger pull than occurs with single-action semiautomatics. Semiautomatic firearms known as double-action/single-action (DA/SA) combine both actions.