The heavy machine gun 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.
Currently, firearms with calibers smaller than 12 mm are generally considered medium machine guns, while those larger than 13-15 mm are generally thought of as autocannons instead of heavy machine guns.
In the late 19th century, Gatlings, and some other externally powered types such as the Nordenfelt were often made in range of calibers, such as half inch and one inch. Thanks to their many barrels, overheating was not so much of issue, and they were also quite heavy.
When Hiram Maxim developed his recoil-powered machine gun that used a single barrel, the first main design was a modest 26 pounds (11.8 kg) and fired a .45-inch rifle-caliber bullet (from a 24-inch barrel). As a famous photo of Maxim himself will attest that even he could pick 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. As a result, he created a water jacket cooling system to enable it to fire for extended periods. This added significant weight, as did the change to more powerful cartridges.
There were two main heavy, rapid-fire weapons, the heavy-caliber, manually powered machine guns and the water-cooled Maxim types. Soon, by the turn of the century, many new designs were developed, some powered by gas or recoil or some combination of the two (Colt 1895, Hotchkiss, etc). Also, rather than the rather heavy water jacket, new designs introduced other types of cooling, such as barrel replacement, metal fins, or heat sinks or some combination of them.
Machine guns diverged into heavier and lighter design. The later model water-cooled Maxim guns and its derivatives the MG 08 and the Vickers gun, as well as the American M1917 Browning machine gun, were all substantial weapons. The .303 Vickers, for example, weighed 33 lb (15 kg) and was mounted on a tripod that brought the total weight to 50 lb (23 kg). The heavier designs could, and in some cases did, literally fire for days on end. The need was to be able to cut down potentially thousands of charging soldiers. The heavy machine gun was mounted on a tripod and was water cooled, and a well-trained and well-supplied crew could fire for hours on end. Carefully positioned heavy machine guns could stop an attacking force before they reached their objectives.
However, during the same period a number of new, lighter air-cooled designs were developed that rather than weighing well over 30 lb (15 kg) were lighter and mobile. In World War I they were to be as important as the heavier designs, and were used to support squads and infantry on the move, on aircraft, and on many types of vehicle as well (and on tanks to some extent). The two that would become critical were new medium and light machine guns. The new medium machine guns offered less, or more difficult to use, cooling than the heavier designs, but more than the lightest.
The lightest of the new designs were not capable of sustained fire, as they did not have an extra cooling features and were fed from a comparatively small magazine. Essentially a machine rifle with a bipod, weapons like the Chauchat or the Madsen 1902 were the most mobile, but were made for single and burst fire. These were used in assaults to great effect by infantry, but were not as popular on other mounts.
The medium designs offered greater flexibility, either using a bipod and being used like lighter designs, or being put on a tripod or on heavier mounts. The Hotchkiss Mark I (e.g., Benet-Mercie, M1909) was a 27.6 lb (12.2 kg) machine gun that normally used a mini-tripod and linkable 30-round strips, but there was also a belt-fed version of it. Not to be confused with heavier Hotchkiss models (such as the M1914), the design proved a useful intermediate and would serve even to the end of WWII in some roles. The design would be followed by lighter machine guns and better medium types.
The Lewis gun, which weighed 27 lb (12.3 kg), was commonly used with a 47-round drum and bipod; it was used on the move in support of squads, and on vehicles and aircraft as well, or on a tripod (either for anti-aircraft use, or to fill in for a heavier machine gun). What made it very useful was that it was significantly lighter than water-cooled weapons, but could fire nearly as much due to a very large cooling assembly. This sort of multipurpose machine gun would be further developed, and later given names like Universal Machine gun (latter called the general-purpose machine gun) and would eventually supplant the water-cooled designs. Later designs have mostly switched to fast barrel replacement for cooling, which further reduces the weapon's weight (but can increase the total weight carried by a soldier). Some earlier designs like the Vickers had this feature, but it was mainly for barrel wear, not for cooling (as they normally used water cooling). It was in the 1920s and 1930s that fast barrel replacement for cooling became more popular (such as the ZB 1930, and later the MG34 and the Bren).
The heavier water-cooled designs continued to be used throughout WWII and into the 1960s, but were gradually phased out in favor of air-cooled designs. The mediums are now used both as heavy machine guns while mounted on tripods and as light machine guns while mounted on bipods. This was possible in part because a heavy, static MG position was not a very effective tactic in vehicle-centered warfare, and the lighter air-cooled designs could nearly match the capabilities of water-cooled designs with a combination of other lighter cooling features. Also, during WWII, many new larger-caliber machine guns were developed, the Soviet Union having developed a number of larger calibers, as well as other countries. (There was the large-caliber Vickers design, for example.)
By the latter half of the 20th century, use of heavy machine guns, especially water-cooled designs, was declining. The venerable Browning M1917 saw its last major use during the 1960s in the Vietnam conflict. At the same time, however, Gatling-type weapons were making a comeback. Those firing 7.62 mm such as the General Electric Minigun were popular for ships, and helicopter mounted weapons, and have established a niche; the Soviet Union also developed a number of Gatling-type weapons. The need for sustained fire on the ground, however, was now nearly entirely the domain of air-cooled medium machine guns that used some cooling manifolds, barrel replacement, and special or heavier barrels. Since there were no more rifle-caliber machine guns (aside from the Gatlings), the term heavy machine gun now mainly just refers to heavy-caliber machine guns. By the 21st century, new heavy-caliber machine guns have become much lighter (for a given type) as well; less than many of the old water-jacketed types.
Heavy machine gun is a term used to describe automatic firearms which are of 7.62mm or 30 caliber or higher. The rounds fired by a heavy machine gun range in size from 7.62mm x 51mm, up to the 50 cal, which is 12.7mm x 99mm. The Russians have a 14.5mm caliber machine gun. However, traditionally the term heavy machine gun is used only for 50 cal.