- A light machine gun or LMG is a machine gun designed to be employed by an individual soldier, with or without an assistant, and as a front-line infantry support. LMGs are often used as squad automatic weapons.
A light machine gun may be identified either by the weapon or by its tactical role. It is used to fire in short 8 -10 round bursts, usually from a bipod; a sustained-fire mount such as a tripod is a characteristic of a medium machine gun. Some machine guns - notably General purpose machine guns - may be deployed as either a light machine gun or a medium machine gun. As a general rule, if a machine gun is deployed with a bipod it is a light machine gun; if deployed on a tripod it is a medium machine gun - unless it has a caliber of about 10mm or larger, making it a heavy machine gun. Modern light machine guns often fire smaller-caliber cartridges than medium machine guns, and are usually lighter and more compact.
- A medium machine gun or MMG, in modern terms, usually refers to a belt-fed automatic firearm firing a full power rifle cartridge.
- The heavy machine gun or HMG is a larger class of machine gun generally recognized to refer to two separate stages of machine gun development. The term was originally used to refer to the early generation of machine guns which came into widespread use in World War I. These fired the standard (~.30 or 7.62mm) rifle cartridge but featured heavy construction, elaborate mountings, and water cooling mechanisms that enabled heavy and sustained defensive fire with excellent accuracy, but with the cost of being too cumbersome to move quickly. 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 is best exemplified by the Maxim gun, invented by American Hiram Maxim. The Maxim was the most ubiquitous machine gun of World War I, regional variants of which were fielded simultaneously by three separate warring nations (Germany with the MG08 in 8mm Mauser, Britain with the Vickers in .303 British, and Russia with the Pulemyot M1910 in 7.62x54R).
The more modern definition refers to a class of large-caliber (generally ~.50 or 12.7mm) machine guns pioneered by John Moses Browning with the M2 machine gun and designed to provide an increased degree of range, penetration and destructive power against vehicles, buildings, aircraft and light fortifications over the standard rifle calibers used in medium or general purpose machine guns. In this sense, the "heavy" aspect of the weapon refers to its superior power and range over light and medium caliber weapons. This class came into widespread use during World War II, when the M2 was used widely in fortifications, on vehicles and in the air by the American forces. A similar HMG capacity was fielded by the Soviets in the form of the DShK in 12.7x108mm. The ubiquitous German MG42, 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 after the D-Day invasion. 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; the M2 is now the oldest serving weapon in the US arsenal, and most nations are equipped with some type of HMG.
