An automatic firearm continuously fires rounds as long as the trigger is pressed and held and there is ammunition in the magazine/chamber. In contrast, a semi-automatic firearm fires one round with each individual trigger-pull.
Although both "semi automatic" and "fully automatic" weapons are "automatic" in the technical sense that the firearm automatically cycles between rounds with each trigger pull, the terms "automatic weapon" and "automatic firearm" are conventionally reserved to describe fully automatic firearms. Use of this convention can avoid confusion. For the mechanisms by which both fully- and semi-automatic weapons cycle, see firearm actions.
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.
- 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 semi-automatics (e.g. 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 non-automatic firearms. Where automatic weapons are permitted, restrictions and regulations on their possession and use may be much more severe than for other firearms. In the United States, taxes and strict regulations affect the manufacture and sale of fully automatic firearms under the National Firearms Act. A prospective user must go through an application process administered by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), which requires a federal tax payment of $200 and a thorough criminal background check. The tax payment buys a revenue stamp, which is the legal document allowing possession of an automatic firearm. The use of a gun trust to register with the BATFE has become an increasingly popular method of acquisition and ownership of automatic firearms.
Other similar weapons not usually called automatic firearms are the following:
- Autocannon, which are 15 mm 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.
- Automatic rifle
- Firearm action
- Bump fire
- Federal Firearms License
- Gun Control Act of 1968
- Gun politics
- Carter, Gregg Lee (2012). Guns in American Society: An Encyclopedia of History, Politics, Culture, and the Law. ABC-CLIO. p. 53. ISBN 978-0-313-38670-1.
- Cutshaw, Charles Q. (28 February 2011). Tactical Small Arms of the 21st Century: A Complete Guide to Small Arms From Around the World. Iola, Wisconsin: Gun Digest Books. p. 250. ISBN 1-4402-2482-X.
- "Secure Gun Trust". https://secureguntrust.com. Retrieved 20 February 2015.