An automatic firearm is a firearm which will keep firing as long as the trigger is being pulled and there is still ammunition in the magazine.
An automatic weapon works by firing, automatically removing the cartridge case from the chamber, ejecting it from the side of the gun, and then loads a new cartridge into the chamber.
An "automatic pistol" or "automatic shotgun" usually means that it has a semi-automatic design. However, fully automatic pistols (for example the Steyr TMP) and shotguns (such as the Daewoo USAS-12 and the AA-12) do exist. "Automatic rifle" usually means that the rifle has a fully automatic design, not semi-automatic.
In most countries, automatic firearms are usually only allowed for the military and police to use. In countries where automatic firearms can be used by the public, there are usually very strict rules on their use.
A fully automatic weapon (a machine gun) is one that keeps firing bullets as long as the trigger is pulled. It stops when the trigger is let go or the ammunition is all gone. In some countries, any weapon that shoots more than one shot at a time when the trigger is pulled once is legally called a machine gun. Some firearms fire more than one bullet after each trigger pull, but only a limited number. This is called burst fire.
Submachine guns are fully automatic weapons that fire a pistol cartridge. They can also be used by only one person. Sometimes they are called machine pistols. 
- U.S. Patent 1,227,897: Automatic gun
- ↑ In United States law, The National Firearms Act of 1934 says that a machine gun is "... any weapon which shoots ... automatically more than one shot, without manual reloading, by a single function of the trigger."
- ↑ Di Maio,Vincent J. M. Gunshot wounds: practical aspects of firearms, ballistics, and forensic techniques, p. 15.