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An automatic firearm is any firearm that will continue to fire so long as the trigger is pressed and held and there is ammunition in the magazine/chamber. While both "semi automatic" and "fully automatic" weapons are "automatic" in technical sense that the firearm automatically cycles between rounds with each trigger pull, under conventional usage a merely semi-automatic firearm is not correctly referred to an "automatic weapon" or an "automatic firearm". The terms "automatic weapon" and "automatic firearm" are conventionally reserved to describe fully automatic firearms. Confusion can be avoided by this convention. A semi-automatic firearm fires one round with each individual trigger pull, while a fully automatic continuously fires rounds whilst the trigger is pressed and held.
The speed of fully automatic firearms is measured in rounds per minute (RPM) or rounds per second (RPS), in what is called the Rate of fire. The speed of fully automatic firearms is compared to each other this way.
Automatic and semi-automatic firearms can be divided into six main categories:
- Assault rifle - the standard type of service rifles in most modern armies, capable of both automatic and semi-automatic fire.
- Battle rifle - a heavier-caliber type of service rifle which some classify as an assault rifle while others see them as a class of their own.
- Automatic shotgun -a type of combat shotgun that is capable of firing shotgun shells automatically, usually also semi-automatically.
- Machine gun - a large group of heavier firearms used for automatic fire of rifle ammunition, usually attached to a mount or supported by a bipod. Depending on size and weight, machine guns are divided into heavy, medium or light machine guns. The ammunition is often belt-fed.
- Submachine gun - an automatic, short rifle (carbine) that uses pistol cartridges. Nowadays seldom used militarily, due to body armour making them ineffective, but they are commonly used by police forces and close protection units in many parts of the world.
- Personal defense weapon - a new breed of automatic firearms that combine the lightness and size of the submachine gun with the heavier-calibre ammunition of the assault rifle, thus in practice creating a submachine gun with body armor penetration capability.
- Machine pistol - a handgun-style firearm, capable of fully automatic or burst fire. They are sometimes equipped with a foldable shoulder stock, to enable better accuracy during automatic fire, which then makes them very similar to submachine guns. Some machine pistols are shaped very similar to their semi-automatics (i.e. the Glock 18). As with SMGs, Machine Pistols generally fire pistol caliber cartridges (such as the 9mm, .40, .45 ACP etc.).
Automatic weapons tend to be restricted to military and police organizations in most developed countries that permit the use of nonautomatic firearms. Where automatic weapons are permitted, restrictions and regulations on their possession and use may be much more severe than for other firearms.
Other similar weapons not usually called automatic firearms are the following:
- Autocannon, which are 15mm or greater in bore diameter or larger and thus considered cannons, not small arms.
- Gatling guns, multiple barrel designs, often used with external power supplies to generate rates of fire higher than automatic firearms.
- Game hunting rifles.
See also 
- Automatic rifle
- Firearm action
- Bump fire
- Federal Firearms License
- Gun Control Act of 1968
- Gun politics
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