An automatic firearm is a firearm that continuously chambers and fires rounds when the trigger mechanism is actuated. The action of an automatic firearm is capable of harvesting the excess energy released from a previous discharge to feed a new ammunition round into the chamber, and then ignite the propellant and discharge the projectile (either bullet, shots or slug) by delivering a hammer/striker impact on the primer.
If both the feeding and ignition procedures are automatically cycled, the weapon will be considered "fully automatic" and will fire continuously as long as the trigger is kept depressed and the ammunition feeding (either from a magazine or a belt) remains available. In contrast, a firearm is considered "semi-automatic" if it only automatically cycles to chamber new rounds (i.e. self-loading) but does not automatically fire off the shot unless the user manually resets (usually by releasing) and re-actuates the trigger, so only one round gets discharged with each individual trigger-pull. A burst-fire firearm is an "in-between" of fully and semi-automatic firearms, firing a brief continuous "burst" of multiple rounds with each trigger-pull, but then will require a manual re-actuation of the trigger to fire another burst.
Although all semi-automatic, burst-fire and fully automatic firearms are "automatic" in the technical sense, the terms "automatic weapon" and "automatic firearm" are used to describe fully automatic firearms. Use of more specific terms such as "full-auto","semi-auto" or "burst" can help avoid confusion. Automatic weapons that can switch between the aforementioned firing modes are known as selective fire weapons.
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 referred to as its cyclic rate. In fully automatic firearms, the cyclic rate is tailored to the purpose the firearm is intended 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, vary with the design of the particular firearm. The MG 34 is a WWII-era machine gun which falls under the category of a "general purpose machine gun". It was manufactured in several variations: with a cyclic rate as high as 1200 rounds per minute, but also in an infantry model which fired at 900 rounds per minute.
Continuous fire generates high temperatures in a firearm's barrel and increased temperatures throughout most of its structure. If fired continuously, the components of the firearm will eventually suffer structural failure. All firearms, whether they are semi-automatic, fully automatic, or otherwise, will overheat and fail if fired indefinitely. This issue tends to present itself primarily with fully automatic fire. For example, the MG34 may have a calculated cyclic rate of 1200 rounds per minute, but is likely to overheat and fail in the space of one minute of continuous fire.
Semi-automatic firearms may also overheat if continuously fired. A semi-automatic firearm typically has an effective firing rate of 40 rounds per minute. Recoil plays a significant role in the time it takes to reacquire one's sight picture, ultimately reducing the effective rate of fire.
Automatic firearms can be divided into six main categories:
Burst mode is typically used to limit the number of rounds fired, due to the inaccuracy of fully automatic fire. In the US M16/M4 platforms, the burst mode fires three rounds per trigger reset. The rifle will not fire again until the trigger is released and 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 referred to as automatic firearms include the following: