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While in United States gun law a "machine gun" is any weapon capable of firing sequential rounds from the same barrel with a single pull of the trigger or any part that renders a weapon capable of such (including auto drop-in sears), the term correctly refers to self-loading support weapons which are designed for sustained automatic fire. A machine gun must fire at least an intermediate round; a fully automatic weapon which fires pistol rounds is either a submachine gun or a machine pistol. An automatic weapon which fires shotgun cartridges is an automatic shotgun. Any weapon which fires rapidly through the use of multiple barrels firing at the same time is a volley gun, not a machine gun.

Light machine guns (LMGs) are designed for portability and short periods of automatic fire from a bipod; modern LMGs usually fire intermediate rounds, though some use full-sized rifle rounds. Obviously, prior to the development of the intermediate round, all LMGs fired rifle rounds. A "Squad Automatic Weapon" or SAW is a typical role for such a gun, though since SAW is a role rather than a proper category of weapon system, it can be said to be fulfilled by any weapon the military chooses to issue as one. Some SAWs are simply assault rifles with extended magazines.

A medium machine gun (MMG) is a weapon designed for protracted automatic fire from a fixed position such as a tripod, using a full-sized rifle round. Almost all modern medium machine guns are belt-fed.

A general purpose machine gun (GPMG) is a weapon combining the capabilities of a light and medium machine gun; usually they are functionally medium machine guns, but take advantage of modern production techniques to enable them to be more portable than their ancestors. GPMGs have largely replaced MMGs in service due to their greater flexibility.

A heavy machine gun (HMG) is usually categorised as a weapon with a caliber greater than .50 in (12.7mm) but less than around 20mm; such weapons are employed in static positions or as vehicle armament. In WW1 and in some cases up to the end of WW2 the term simply referred to any machine gun designed to be emplaced and was largely defined by sheer physical weight, meaning some sources will describe weapons as HMGs which fit the modern definition of an MMG because they were defined as such at the time.

Non-rotary automatic weapons of calibers greater than 20mm are usually categorised as autocannons; in such weapons, automatic fire is the only qualifier, the feeding method can vary from an electrically driven belt to large stripper clips as with the Bofors 40mm. Revolver cannons use a cylinder mechanism as part of their loading method, while chainguns use a power-assisted bolt driven by a chain.

An automatic weapon which fires low-velocity explosive rounds is usually categorised as a grenade machine gun (GMG) or automatic grenade launcher (AGL).

Rotary weapons are usually treated as their own sub-class rather than using the light / medium / heavy descriptors. These are referred to as rotary or Gatling guns; it is never correct to call a rotary gun a chaingun. Strictly speaking, most rotary gun designs are not machine guns or even automatic, since they use external power from either a crank or motor to drive a fundamentally manually-operated weapon, but since they are used in the same role of producing sustained fire, they are categorised as machine guns.