An automatic firearm continuously fires rounds as long as the trigger is pressed or held and there is ammunition in the magazine/chamber. In contrast, a semi-automatic firearm fires one round with each individual trigger-pull.
Although all "semi-automatic", "burst fire", and "fully automatic" firearms 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 by firearm enthusiasts to describe fully automatic firearms. Use of the terms "fully automatic" or "full auto" can avoid confusion. Firearms are further defined by the type of firearm action used.
Rates of fire
Self-loading firearms are designed with varying rates of fire due to having different purposes. The speed with which a self-loading firearm can cycle through the functions of:
is called the cyclic rate. In fully automatic firearms the cyclic rate is tailored to the purpose that the gun is made to serve. Anti-aircraft machine guns often have extremely high rates of fire to maximize the probability of a hit. In infantry support weapons these rates of fire are often much lower and in some cases variable within the design of the firearm. The MG 34 is a WWII-era machine gun which today would be referred to as a general purpose machine gun. It came in several variations with a cyclic rate as high as 1200 rounds per minute, but also made an infantry model which fired at 900 rounds per minute. 
Effective rate of fire
Firing any firearm generates a very high temperature in the firearm's barrel and elevated temperature throughout much of its structure. If fired too fast, the components of the firearm will suffer a structural failure. This means that all firearms, regardless of whether they are semi-automatic, fully automatic, or burst mode in their firing methods, will overheat and fail if fired too often. This is especially a problem with fully automatic fire. In actual use (for example the MG34), a gun might be able to fire at 1200 rounds per minute, but in one minute it may also overheat and fail. So guns used in a repeated firing mode must not be fired too often. The MG34 is fired manually in bursts of 5 to 7 rounds (no automatic disconnector mode in this gun). It can fire at an effective rate of 150 rounds per minute.
Similarly semi-automatic firearms will also overheat if not allowed to cool. A semi-automatic rifle typically has an effective firing rate of 40 rounds per minute. A large part of the reason that this is so low, is that the recoil of firing a round pushes the gun's aim off target. The time it takes to "reacquire" the target slows the effective firing rate. The Army Study Guide lists the sustained rate of fire for an M4 Rifle at 12 to 15 rounds per minute.
Full-automatic firearm types
Automatic firearms can be divided into six main categories:
- Automatic rifle
- The standard type of service rifles in most modern armies, usually capable of selective fire. Assault rifles are a specific type of select-fire rifle chambered in an intermediate cartridge and fed via a high-capacity detachable magazine. Battle rifles are similar, but chambered in a full-powered cartridge.
- 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 suppressive automatic fire of rifle ammunition, usually attached to a mount or supported by a bipod. Depending on size, weight and role, 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. Today 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 medium power calibre ammunition of 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 fire pistol caliber cartridges (such as the 9mm, .40, .45 ACP etc.).
Burst mode is often used in military firearms to limit the number of rounds fired due to the inaccuracy of fully automatic fire. In the US M16/M4, rifles for example, the burst mode is three rounds. Pulling and holding the trigger results in three rounds being fired. The gun will not fire again until the trigger is released and then pulled again. There are suggestions that fully automatic fire has no genuine benefit and has been restricted or banned in combat due to being a waste of ammunition. The M4 carbine is now the main combat rifle of the US armed forces and has been available until recently in semi-automatic and burst mode of three rounds only.
Automatic weapons tend to be restricted to military and police organizations in most developed countries that permit the use of semi-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 ATF 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.
- "Maschinengewehr Modell 34 (MG34) General Purpose Machine Gun (GPMG)". militaryfactory.com. Retrieved 3 March 2017.
- "M240B - Machine Gun". armystudyguide.com. Retrieved 3 March 2017.
- "Maintaining a Sustained Rate or Fire". thefirearmblog.com. Retrieved 3 March 2017.
- "M4 - 5.56 mm Semiautomatic Rifle". armystudyguide.com. Retrieved 3 March 2017.
- 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.
- "Full Auto: Battlefield Necessity or A Waste of Ammo?". military.com. Retrieved 3 March 2017.