The invention of a military firearm that could produce rapid, repeating fire to overwhelm and repel an attacking force or act as an offensive or defensive force multiplier had been sought for nearly 900 years. Early attempts to invent what today is known as the machine gun did not produce a fully automatic weapon but resulted in often bulky and semi-reliable guns consisting of single shot barrels gathered together and mounted on a gun carriage or tripod.
A machine gun is a fully automatic, rifled auto-loading firearm designed for sustained direct fire with rifle cartridges. Other automatic firearms such as automatic shotguns and automatic rifles (including assault rifles and battle rifles) are typically designed more for firing short bursts rather than continuous firepower.
As a class of military kinetic projectile weapons, machine guns are designed to be mainly used as infantry support weapons and generally used when attached to a bipod or tripod, a fixed mount or a heavy weapons platform for stability against recoils. Many machine guns also use belt feeding and open bolt operation, features not normally found on other infantry firearms.
The first successful machine-gun designs were developed in the mid-19th century. The key characteristic of modern machine guns, their relatively high rate of fire and more importantly mechanical loading,first appeared in the Model 1862 Gatling gun, which was adopted by the United States Navy. These weapons were still powered by hand; however, this changed with Hiram Maxim’s idea of harnessing recoil energy to power reloading in his Maxim machine gun.
Innovation and necessity carried the machine gun even further and with a second world war beginning in 1939, new models and improvements to existing guns were widespread. Germany produced the MG-34 and MG-42. Modern versions of the MG-42 are still in use today with many militaries.
The technological advances made during World War II and the advent of the Cold War (1947-1991) influenced further machine gun refinements throughout the world.
Unlike semi-automatic firearms, which require one trigger pull per round fired, a machine gun is designed to continue firing for as long as the trigger is held down.
Some machine guns have in practice sustained fire almost continuously for hours; other automatic weapons overheat after less than a minute of use. Because they become very hot, the great majority of designs fire from an open bolt, to permit air cooling from the breech between bursts. They also usually have either a barrel cooling system, slow-heating heavyweight barrel, or removable barrels which allow a hot barrel to be replaced.
Most modern machine guns are of the locking type, and of these, most utilize the principle of gas-operated reloading, which taps off some of the propellant gas from the fired cartridge, using its mechanical pressure to unlock the bolt and cycle the action.