A machine gun is a fully-automatic firearm that is capable of firing bullets in rapid succession. Above a certain caliber, such automatic weapons are generally referred to as autocannons.
Unlike semi-automatic firearms, which require one trigger pull per bullet fired, a machine gun will continue to fire bullets as long as the trigger is held down. Although the term "machine gun" is often used to describe all fully-automatic weapons, a distinction can be made based on the size of the bullet used, and whether the cartridge is "fired" from a positively locked, closed bolt, or a non-positively locked, open bolt. A fully-automatic firearm with a bullet caliber of more than 12.7mm (0.5 inch) is called an automatic cannon while smaller caliber open bolt firearms would still be called "machine guns". Closed bolt fully-automatic pistols would be called "assault pistols", closed bolt selective fire rifles would together be called "assault rifles", and so on.
The machine gun's primary role in ground combat is to provide suppressing fire on an opposing force's position. This forces the enemy to take cover. This either halts an opposing offensive, or allows friendly forces to move onto the field with less danger.
To this end, most light machine guns are not designed for repeatability of aim or accuracy. Most are designed with a small degree of inaccuracy, to lay down a field of fire. This is referred to as the "cone" of fire, because the rounds spread out as they travel. Light machine guns usually have simple iron sights. A common aiming system, favored by the Israelis, is to alternate solid and tracer rounds, so shooters can walk the fire into the target, and direct other soldiers' fire.
Assault rifles are a compromise between the light machine gun and the traditional soldier's rifle, allowing single-shot, burst and (sometimes) full-automatic fire options. See that article for more details.
Many heavy machine guns , such as the M2 0.50 Cal machine guns are so accurate that they can actually be used to snipe targets at great distances. Some models have been equipped with aim points that can be preset.
All machine guns require the following components:
- A feed system to load the firing chamber. Cartridges can be fed into the chamber by a variety of methods, the most common being spring-fed magazines or ammunition belts.
- A trigger mechanism to fire the round. This includes the actual trigger, a trigger sear to catch the bolt, a bolt and a firing pin, as well as other components. Typically, the act of pulling the trigger causes something to strike the primer on the round in the chamber, and disengages the sears. This allows continual cycling of the bolt until the trigger is released. A sear then grabs the bolt or firing pin. This stops the machine gun at some point in its cycle.
- An extractor system to eject the spent or misfired cartridge. Usually this is fairly simple. A pin on the side of the bolt catches a ridge on the cartridge and flicks it out an ejection port.
These components form a mechanism which must be powered by something. If powered by a spring absorbing the recoil of a fired cartridge, it is called recoil-operated. If powered by the expanding gases of a fired cartridge, it is called gas-actuated. If it powered by an external force, such as a motor, it is called a chain gun.
All machine guns follow a cycle:
- Removing the spent cartridge through an ejection port.
- Cocking the trigger mechanism so the weapon can be fired again.
- Loading the next round into the firing chamber. Usually spring tension or a cam forces the new round and bolt back into the firing chamber.
A mechanism makes the firing pin fire the cartridge, activating the ejection and reloading steps. The cycle repeats. This full cycle takes a fraction of a second, and can thus occur many times per second. The operation is basically the same, regardless of the means of activating these mechanisms. Some examples:
- A recoil-actuated machine gun uses the recoil to first unlock, and then operate the action. Heavy machine guns, like the M2 .50 and Browning .50 are of this type. These can be recognized by a large cocking lever needed to feed the first round.
- An externally actuated machine gun uses an outside power source, such as an electric motor or even a hand crank to move its mechanism through the firing sequence. Most modern weapons of this type are called chain guns in reference to their driving mechanism. Gatling guns and revolver cannon have several barrels or chambers on a rotating carousel, and a system of cams that load, cock, and fire each mechanism progressively as it rotates through the sequence. The continuous nature of the rotary action allows for an incredibly high cyclic rate of fire, often several thousand rounds per minute. Not all chain guns use multiple barrels or chambers, though. Chain guns are less prone to jamming than a gun operated by gas or recoil, as the external power source will eject misfired rounds with no further trouble. This is not possible if the force needed to eject the round comes from the round itself. Chain guns are generally used with large shells, 20 mm in diameter or more, though some, such as the M134 Minigun, fire smaller cartridges. They offer benefits of reliability and firepower, though the weight and size necessary for the driving mechanism makes them impractical for use outside of a vehicle or aircraft mount.
Heavy machine guns are often water-cooled, or have interchangeable barrels which must be changed periodically to avoid overheating. The higher the rate of fire, the more often barrels must be changed and allowed to cool. To minimize this, most air-cooled guns are fired only in short bursts, or at a reduced rate of fire.
Not all machine guns strike the primer in the same way. In blowback machine guns, the act of seating the round also fires the round. In gas operated and recoil-operated guns, a separate step in the firing sequence is needed to strike the round. In progressive-fire guns, the firing pin is cycled by cams. In some automatic cannon, the primer is fired electrically.
In weapons where the round seats and fires at the same time, mechanical timing is essential for operator safety, to prevent the round from firing before it is seated properly. This is especially important in weapons like the 40mm grenade launcher, where high explosives are present in the rounds being fired.
Machine guns are controlled by one or more mechanical sears. When a sear is in place, it effectively stops the bolt at some point in its range of motion. Some sears stop the bolt when it is locked to the rear. Other sears stop the firing pin from going forward after the round is locked into the chamber.
Almost all weapons have a "safety" sear, which simply keeps the trigger from engaging.
Multi-shot guns have a long development, going as far back to the 1400s with plans from Leonardo Da Vinci, and stretching back to some of the earliest firearms, and attempts at higher rates of fire and some machine gun like traits happened as early as the 1700s. However, It would not be until the mid-1800s successful machine guns designs came into existence. Key characteristic of modern machine guns, their relatively high rate of fire and more importantly machine (automatic) loading, came with 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. Mr. Gatling also experimented with electric motor powered gatling guns, this externally powered machine reloadeding has seen use in modern weapons as well. The Vandenburg and Miltrailleuse volley guns concepts has been revived partially in the early 21st century in the form of electronically controlled mult-barrelled volley guns. It is important to note that what exactly constitues a machine gun, and whether volley guns are a type of machine gun, and to what extent some earlier types of devices are consider to be like machine guns is a matter of debate in many cases and can vary depending which language and exact definition is used.
Early rapid-firing weapons
Among first known ancestor of multi-shot weapons was created by James Puckle, a London lawyer, who patented what he called "The Puckle Gun" on May 15, 1718. It was a design for a 1 in. (25.4mm) caliber flintlock semi-automatic cannon able to fire 9 rounds before reloading, intended for use on ships. According to Puckle, it was able to fire round bullets at Christians and square bullets at Turks. While ahead of its time, foreshadowing the designs of revolvers, it was not adopted or produced.
In the early and mid-19th century, a number of rapid-firing weapons appeared which offered mult-shot fire, a number of semi-automatic weapons as well as volley guns. Volly guns (such as the Mitrailleuse) and double barrel pistol relied on duplicating all the parts of gun. Pepperbox pistols did away with needing multiple hammers, but used multiple barrels. Revolvers further reduced this to only needing a pre-prepared magazine using the same barrel and ignitions. However, like the Puckle gun, they were still only semi-automatic.
The coffee-mill machine gun of the civil war featured both automatic loading and single barrel, only separated from modern machine gun functionally by being hand-powered rather than using cartridges.
The Gatling gun, patented in 1861 by Richard Jordan Gatling was the first machine gun to offer controlled sequential automatic fire with automatic loading. The design's key features were machine loading of prepared cartridges and a hand operated crank for sequential high-speed firing. It first saw very limited action in the American Civil War and was subsequently improved. Many were sold to other armies in the late 1800s and continued to be used into the early 1900s, until they were gradually supplanted by Maxim-type machine guns. The gatling weapons were the first widely used machine guns, and, due to their mutiple barrels, could offer more sustained fire then the first generation of air-cooled recoil operated machine guns. The weight, complexity, and resulting cost of the multi-barrel design meant that recoil-operated weapons, which would be could be made lighter and cheaper, would supplant them. It would be another 50 years before the concept was again used to allow extemely high rates of fire, such as in chain guns, miniguns, and automatic aircraft cannons.
The first true machine gun was the Maxim machine gun, invented in 1883 by Hiram Maxim. It used the recoil energy 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. Maxim's other great innovation was the use of water cooling (via a water jacket around the barrel) to reduce overheating. Maxim's gun sparked a cascade of new designs and was widely adopted. The design required less crew, was lighter, and more useable than earlier Gatling guns.
Heavy guns such as the Vickers machine gun were joined by many other machine weapons, which mostly had their start in the early 20th century. Submachine guns (e.g. the Thompson submachine gun, or 'Tommy gun') as well as lighter machine guns (the BAR for example) saw their first major use in WW1 along with heavy use of large-caliber machine guns. Design features of machine guns were applied to automatic handguns, "machine pistols", such as the Luger (although these did not yet have full automatic fire). Machine guns were mounted in aircraft for the first time in World War I. Firing through a moving propeller was solved in a variety of ways, including the interrupter gear, metal reinforcement of the propeller or simply avoiding the problem with wing mounted guns.
During the inter-war years, many new designs were developed, such as the Browning 50-caliber, in 1933, which, along with the others were used in World War II. The trend toward automatic rifles, lighter machine guns, and more powerful submachine guns culminated in the invention of Germany's revolutionary MP44 assault rifle, a weapon that combined characteristics of an ordinary rifle and a machine gun. Many aircraft were equipped with machine cannons, and similar cannon (nicknamed "pom-pom guns") were used as anti-aircraft weapons. The designs of Bofors of Sweden were widely used by both sides and have greatly influenced similar weapons developed since then.
The Cold War era saw mostly a refinement of weapon types in the form of lower weight and higher reliability. The semi-automatic rifles of World War II vintage were almost totally replaced by lighter assault rifles such as the M-16 and Soviet AK-74. Infantry adopted general-purpose machine guns like the American M-60 for squad use, using air cooling for lighter weight. Heavy machine guns were retained for ground vehicles and fortifications. For aircraft use, even heavy machine guns proved to lack killing power in the air-to-air role, and by the late 1950s fighter aircraft armament had almost totally switched to automatic cannons. Machine guns, with lower recoil, remained popular for helicopters and for ground attack aircraft, supplemented by new, Gatling-style electric multi-barrel weapons like the American Minigun. In police, special operations, and other paramilitary roles, smaller automatic weapons, including light submachine guns and machine pistols, proliferated, many relying on the increasingly ubiquitous 9x19mm round.
The development of conventional machine guns has been slowed by the fact that existing designs of machine guns are adequate for most purposes, although significant developments are taking place with regard to anti-armour and anti-missile weapons.
In the future, electronically controlled machine guns with ultra-high rates of fire may see use in some applications, although current small-caliber weapons of this type have found little use: they are too light for anti-vehicle use, but too heavy (especially with the need to carry a tactically useful amount of ammunition) for individual soldiers. The trend towards higher reliability and lower mass for a given power will likely continue.
The newest machine-gun designs center around the Personal Defense Weapon concept, a cross between a machine gun and a sidearm that is useful for support personnel who need a small, concealable weapon that keeps their hands free, yet provides massed firepower when needed -- roles such as security teams in occupation zones. A precursor is the now-popular FN P90. The Pentagon is working on a next-generation infantry weapon code-named XM29, which will combine a lightweight ceramic body carbine with airburst grenade launcher, digitized target acquisition, night vision, and zoom scope.
Machine Gun is also the name of a free jazz album by Peter Brötzmann as well as the New York based improvising band Machine Gun which featured Thomas Chapin and Sonny Sharrock.