Medium machine gun

A medium machine gun or MMG, in modern terms, usually refers to a belt-fed automatic firearm firing a full-power rifle cartridge and typically weighs from 15 to 40 pounds (6.8 to 18.1 kg). MMGs usually have some type of provision for extended firing, such as a removable or extra-heavy barrel, cooling fins, or a water cooling jacket, but are light enough to be used with a bipod. They occupy a gray area between light machine guns and heavy machine guns.
Two features which have remained somewhat constant, however, are some added ability for greater fire over automatic rifles, and the ability to be used in both light infantry support roles on a bipod, but also on mounts and tripods. While heavy machine guns (HMG) are mostly fired from heavy mounts, and light machine guns (LMG) are usually operated with bipods, MMGs have historically been used in both. Heavy machine guns are either crew-served or mounted, while MMGs are usually operated by one to two soldiers. Light machine guns and automatic rifles are often an individual weapon with a fixed, naked barrel. On the other hand medium machine guns have usually had more endurance of some sort, such as the aforementioned heavier barrel, barrel-change, fins etc.

In the late 1800s, Gatling gun and other externally-powered types such as the Nordenfelt were often made in different ranges 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; being, essentially, heavy machine guns.
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) in weight, and fired a .45-inch rifle caliber bullet (from a 24 inch long barrel). As depicted in a famous photo of Maxim, it could be picked up complete with its 15 pound (6.8 kg) tripod with one arm. It was similar to present day (2005) 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 changes to more powerful cartridges. This class of heavy water-cooled machines gun would eventually be regarded as the classic heavy machine guns. However, the much lighter total weight possible by using recoil to power automatic loading was not lost on the firearms designers of the day. Soon there was a host of new automatic firearms that used this concept, such as the Borchardt pistol, the Cei-Rigotti rifle, the Madsen 1902, as well as lighter, gas-operated, air-cooled designs.