A machine gun is a mounted or portable firearm that operates fully automatically. Typically, it is designed to discharge rifle cartridges rapidly in succession from either an ammunition belt or a large-capacity magazine, often at several hundred rounds per minute. Machine guns were manually operated in earlier iterations, requiring actions such as turning a hand crank. According to United States law, the term “machine gun” specifically refers to any fully automatic firearm. In the context of U.S. legislation, a machine gun is defined by The National Firearms Act of 1934 as “… any weapon which shoots … automatically more than one shot, without manual reloading, by a single function of the trigger.”

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There exist two main definitions of the upper limit of caliber for machine guns:

1. caliber larger than 12.7 millimeters (mm) (.50 caliber)
2. caliber larger than 20 mm

Larger-caliber automatic weapons are typically termed autocannons. However, there are instances where weapons around 15mm have been called either autocannons or machine guns, depending on their characteristics. Additionally, the distinction may be influenced by whether the gun fires conventional or explosive rounds.

Automatic weapons firing large-caliber explosive rounds are often classified as autocannons or automatic grenade launchers. Machine guns are characterized by a high ratio of barrel length to caliber, featuring an extended barrel for a relatively small caliber.

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