A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rifle cartridges in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute. The first weapon that falls under this definition was the Maxim Gun, invented by Sir Hiram Maxim in 1884. In United States law, machine gun is a term of art for any fully-automatic firearm, and also for any component or part that will modify an existing firearm into a fully-automatic firearm.

Machine guns are generally categorized as submachine guns, machine guns, or autocannons. The distinction between submachine guns and machine guns is subtle, hinging upon whether the ammunition used is intended for use in pistols (chiefly semi-automatic pistols) or rifles; the difference between machine guns and autocannons is based on caliber, with autocannons using calibers larger than 16 mm. Another factor is whether the gun fires conventional rounds or explosive rounds.

Guns firing large-caliber explosive rounds are generally considered either autocannons or automatic grenade launchers ("grenade machine guns"). By contrast to the other two categories (submachine guns and autocannons), machine guns (like rifles) tend to share a very high ratio of barrel length to caliber (a long barrel for a small caliber); indeed, a true machine gun is essentially a fully-automatic rifle, and the boundaries between the two are often blurred. Often, the criterion for a machine gun as opposed to an automatic rifle is considered to be the presence of a quick change barrel or other cooling system (see below).

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