Machine gun

A machine gun is a fully automatic mounted or portable firearm designed to fire bullets in quick succession from an ammunition belt or magazine, typically at a rate of 300 to 1800 rounds per minute. Note that not all fully automatic firearms are machine guns. Submachine guns, rifles, assault rifles, shotguns, pistols or cannons may be capable of fully automatic fire, but are not designed for sustained fire. As a class of military rapid-fire guns, true machine guns are fully automatic weapons designed to be used as support weapons and generally used when attached to a mount or fired from the ground on a bipod or tripod. Many (though by no means all) machine guns also use belt feeding and open bolt operation, features not normally found on rifles.

The first practical self-powered machine gun was invented in 1884 by Sir Hiram Maxim. The "Maxim gun" used the recoil power of the previously fired bullet to reload rather than being hand-powered, enabling a much higher rate of fire than was possible using earlier designs such as the Nordenfelt and Gatling weapons. Maxim also introduced the use of water cooling, via a water jacket around the barrel, to reduce overheating. Maxim's gun was widely adopted and derivative designs were used on all sides during the First World War. The design required fewer crew and was lighter and more usable than the Nordenfelt and Gatling guns. First World War combat experience greatly increased the importance of the machine gun. The United States Army issued four machine guns per regiment in 1912, but that allowance increased to 336 machine guns per regiment by 1919.

Heavy guns based on the Maxim such as the Vickers machine gun were joined by many other machine weapons, which mostly had their start in the early 20th century such as the Hotchkiss machine gun. Submachine guns (e.g., the German MP 18) as well as lighter machine guns (the Lewis machine gun, for example) saw their first major use in the First World War, along with heavy use of large-caliber machine guns. The biggest single cause of casualties during this time was actually artillery, but combined with wire entanglements, machine guns earned a fearsome reputation. The automatic mechanisms of machine guns were applied to handguns, giving rise to automatic pistols (and eventually machine pistols) such as the Borchardt (1890s) and later submachine guns (such as the Beretta 1918).

Machine guns were mounted in aircraft for the first time during the First World War. Firing "through" a moving propeller was initially evaded through metal reinforcement of the propeller, or simply avoiding the problem with wing-mounted guns or having a pusher propeller. By the late spring of 1915, the introduction of the gun synchronizer by the German Fliegertruppe made it possible to fire forward through a spinning propeller for fighter aircraft to come into their own, also requiring the ordnance used to have a closed bolt firing cycle for the best chance of success.