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 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).

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 500 grain, but below 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 (Maschinengewehr 18 Tank und Flieger, 'Machinegun 18 Tank and Aircraft') 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 are.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) are heavy machine guns.

Autocannons or "cannons" are defined by any gun firing a cartridge above .80 caliber (i.e., 20 mm), except for 40mm specifically. Anything under .80 caliber is considered a machine gun.

In the late 19th century, Gatling guns and other externally powered types such as the Nordenfelt gun and Gardner gun were often made in a variety of calibers, such as 0.5-inch and 1-inch. Due to their multiple barrels, overheating was not so much of an issue, but they were also quite heavy. When Maxim developed his recoil-powered Maxim gun using a single barrel, his first main design weighed a modest 26 pounds (11.8 kg) and fired a .45-inch rifle-caliber bullet from a 24-inch barrel. A famous photo of Maxim showed him picking it up by its 15-pound tripod (6.8 kg) with one arm. It was similar to present-day medium machine guns, but it could not be fired for extended periods due to overheating. As a result, Maxim created a water jacket cooling system to enable it to fire for extended periods. However, this added significant weight, as did the change to more powerful rifle cartridges.

There were thus two main types of heavy, rapid-fire weapons: the manually powered, multiple-barrel machine guns and the single-barrel Maxim guns. By the end of the 19th century, many new designs such as the M1895 Colt–Browning and Hotchkiss M1897 were developed, powered by gas operation or recoil operation. Also, rather than the heavy water jacket, new designs introduced other types of barrel cooling, such as barrel replacement, metal fins, heat sinks or some combination of these.