thumb|upright=1.35|The [[M2 Browning machine gun with a tripod weighs 58 kg (128 lb).]]

A heavy machine gun (HMG) is significantly larger than light, medium or general-purpose machine guns. HMGs are typically too heavy to be man-portable (carried by one person) and require mounting onto a weapons platform to be operably stable or tactically mobile, have more formidable firepower, and generally require a team of personnel for operation and maintenance.

There are two classes of weapons generally defined as HMGs:

  • The historical definition refers to machine guns, typically chambered in standard full-power cartridges, that are identified as being "heavy" due to their weight and cumbersomeness, which prevents infantrymen from transporting them on foot. Examples include the Maxim machine gun and M1917 Browning machine gun.
  • The modern definition refers to "heavy caliber" machine guns, pioneered by the German Empire's MG 18 TuF which was a Maxim derivative chambered in 13.2×92mmSR fielded near the end of World War I. They are designed to provide increased effective range, penetration and stopping power against vehicles, aircraft and light fortifications beyond the full-power cartridges used in battle rifles and medium or general-purpose machine guns, and far beyond the intermediate cartridges used in assault rifles, light machine guns and squad automatic weapons. HMGs also have more felt recoil than its lighter counterparts. Popular HMG rounds today are the 12.7×99mm, 12.7×108mm and 14.5×114mm cartridges.

Classification

The term originally referred to the generation of machine guns which came to prominence in the lead up to and during World War I. These fired standard full-power rifle cartridges such as the 7.92×57mm Mauser, 7.7×56mmR (.303 British) or 7.62×54mmR, but featured heavy construction, elaborate mountings and water-cooling mechanisms that enabled long-range sustained automatic fire with excellent accuracy. However, these advantages came at the cost of being too cumbersome to move and required a crew of several soldiers to operate them. Thus, in this sense, the "heavy" aspect of the weapon referred to the weapon's bulk and ability to sustain fire, not the cartridge caliber. This class of weapons was best exemplified by the Maxim gun, invented by the American inventor Hiram Maxim. The Maxim was the most ubiquitous machine gun of World War I, variants of which were fielded in large simultaneously by three separate warring nations—Germany with the MG 08, Britain with the Vickers, and Russia with the PM M1910).

thumb|upright|left|A [[Ukrainian Ground Forces soldier firing the DShKM in heavy role.]]

The modern definition refers to a class of machine guns chambered in "heavy caliber" ammunition, generally with a minimum bullet diameter of 12mm, a minimum cartridge case length of 80mm and a minimum bullet weight of , but a bullet diameter of 20mm which are considered "medium caliber" ammunition for autocannons. Pioneered by the German Empire's 13.2×92mmSR caliber MG 18 TuF () during World War I, these weapons are designed to provide increased range, penetration and destructive power against vehicles, buildings, aircraft and light fortifications beyond the standard rifle calibers used in medium or general-purpose machine gun, or the intermediate cartridges used in light machine guns. In this sense, the "heavy" aspect of the weapon refers to its superior power and range over light and medium caliber weapons, in addition to its weight. This class of machine gun came into widespread use during World War II, when the M2 was used widely in fortifications, on vehicles and in aircraft by American forces. A similar HMG capacity was later fielded by the Soviets in the form of Vasily Degtyaryov's DShK in 12.7×108mm. The ubiquitous German MG42 general-purpose machine gun, though well-suited against infantry, lacked the M2's anti-fortification and anti-vehicle capability, a fact that was noted and lamented by the Germans. The continued need for a longer-range machine gun with anti-materiel capability to bridge the gap between exclusively anti-infantry weapons and exclusively anti-materiel weapons has led to the widespread adoption and modernization of the class, and most nations' armed forces are equipped with some type of HMG.

Currently, machine guns with calibers from .22 to .250 caliber (i.e., 5.45 mm to 6 mm) are considered light machine guns; medium machine guns have .264 to .33 caliber (i.e., 6.5 mm to 8 mm); and .50 caliber or larger (i.e., 12.7 mm to 21 mm) for heavy machine guns.

Autocannons, or "cannons" are defined by any gun firing a cartridge above .80 caliber (i.e., 20 mm), but does not apply to 40mm grenade launchers. Anything else under .80 caliber is considered a machine gun.