A machine gun is a fully automatic mounted or portable firearm, usually designed to fire bullets in quick succession from an ammunition belt or magazine, typically at a rate of several hundred rounds per minute.
Machine guns are generally categorized as submachine guns, machine guns, or autocannons. Submachine guns are hand-held small portable automatic weapons for personal defense or short-range combat firing pistol-caliber rounds. A machine gun is often portable to a certain degree, but is generally used when attached to a mount or fired from the ground on a bipod, and generally fires a rifle cartridge. Light machine guns are small enough to be fired and are hand-held like a rifle, but are more effective when fired from a prone position. The difference between machine guns and autocannons is based on caliber, with autocannons using calibers larger than 16 mm.
Another factor is whether the gun fires conventional rounds or explosive rounds. Guns firing large-caliber explosive rounds are generally considered either autocannons or automatic grenade launchers ("grenade machine guns"). In contrast to submachine guns and autocannons, machine guns (like rifles) tend to share the characteristic of a very high ratio of barrel length to caliber (a long barrel for a small caliber); indeed, a true machine gun is essentially a fully automatic rifle, and often the primary criterion for a machine gun as opposed to an automatic rifle is the presence of a quick-change barrel or other cooling system (see below).
In United States gun law, machine gun is a technical term for any fully automatic firearm, and also for any component or part that will modify an existing firearm such that it functions as a fully automatic firearm.
Overview of modern automatic machine guns 
Unlike semi-automatic firearms, which require one trigger pull per round fired, a machine gun is designed to fire as long as the trigger is held down. Nowadays the term is restricted to relatively heavy weapons fired from some sort of support rather than hand-held, able to provide continuous or frequent bursts of automatic fire for as long as ammunition lasts. Machine guns are normally used against unprotected or lightly protected personnel, or to provide suppressive fire.
Some machine guns have in practice maintained suppressive fire almost continuously for hours; other automatic weapons overheat after less than a minute of use. Because they become very hot, practically all machine guns fire from an open bolt, to permit air cooling from the breech between bursts. They also have either a barrel cooling system, or removable barrels which allow a hot barrel to be replaced.
Although subdivided into "light", "medium", "heavy" or "general-purpose", even the lightest machine guns tend to be substantially larger and heavier than other automatic weapons. Squad automatic weapons (SAW) are a variation of light machine gun and require only one operator (sometimes with an assistant to carry ammunition). Medium and heavy machine guns are either mounted on a tripod or on a vehicle; when carried on foot, the machine gun and associated equipment (tripod, ammunition, spare barrels) require additional crew members.
According to U.S. Army doctrine, a machine gun is distinguished from an automatic rifle by how it is used: a machine gun is a crew-served weapon, while an automatic rifle is used by a single person. While most weapons are designed to be used exclusively in one manner or the other, FM 3-22.68 "Crew-Served Machine Guns", describes how the M249 can be used either as a machine gun or as an automatic rifle: "Both the M249 automatic rifle and the M249 machine gun are identical, but its employment is different. The M249 automatic rifle is operated by an automatic rifleman, but its ammunition may be carried by other Soldiers within the squad or unit. The M249 machine gun is a crew-served weapon."
Other automatic weapons are subdivided into several categories 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. Full automatic firearms using pistol-caliber ammunition are called machine pistols or submachine guns largely on the basis of size. Selective fire rifles firing a full-power rifle cartridge from a closed bolt are called automatic rifles or battle rifles, while rifles that fire an intermediate cartridge (see below) are called assault rifles. The difference in construction was driven by the difference in intended deployment. Automatic rifles (such as the Browning Automatic Rifle) were designed to be a high duty cycle arm for support of other troops, and were often made and deployed with quick change barrel assemblies to allow quick replacement of over heated barrels to allow for continued fire, and may have been operated by both the person actually firing the weapon as well as an additional crewman to assist in providing and caring for ammunition and the barrels, similar to a reduced version of a squad weapon (above). The assault rifle generally was made for a more intermittent duty cycle, and was designed to be easily carried and used by a single person.
Assault rifles are a compromise between the size and weight of a pistol-caliber submachinegun and a full size traditional automatic rifle by firing intermediate cartridges, (or sometimes full power cartridges) and allowing semi-automatic, burst or full-automatic fire options (selective fire), often with two or more of these available on the rifle at once.
In certain states, like California, certain weapons that "cosmetically" resemble true assault rifles, but are only semi-automatic (autoloading), are categorized as "assault weapons" and possession by civilians is generally illegal. Supporters of gun rights generally consider this application of the phrase "assault weapon" to be a misnomer and this term is in fact seldom used outside of the United States for these civilian firearms.
The machine gun's primary role in modern ground combat is to provide suppressive fire on an opposing force's position, forcing the enemy to take cover and reducing the effectiveness of his fire. This either halts an enemy attack or allows friendly forces to attack enemy positions with less risk.
Light machine guns usually have simple iron sights. A common aiming system is to alternate solid ("ball") rounds and tracer ammunition rounds (usually one tracer round for every four ball rounds), so shooters can see the trajectory and "walk" the fire into the target, and direct the fire of other soldiers.
Many heavy machine guns, such as the Browning M2 .50 caliber machine gun, are accurate enough to engage targets at great distances. During the Vietnam War, Carlos Hathcock set the record for a long-distance shot at 7382 ft (2250 m) with a .50 caliber heavy machine gun he had equipped with a telescopic sight. This led to the introduction of .50 caliber anti-materiel sniper rifles, such as the Barrett M82.
All machine guns follow a cycle:
- Pulling (manually or electrically) the bolt assembly/bolt carrier rearward by way of the cocking lever to the point bolt carrier engages a sear and stays at rear position until trigger is activated making bolt carrier move forward
- Loading fresh round into chamber and locking bolt
- Firing round by way of a firing pin or striker (except for aircraft medium caliber using electric ignition primers) hitting the primer that ignites the powder when bolt reaches locked position.
- Unlocking and removing the spent case from the chamber and ejecting it out of the weapon as bolt is moving rearward
- Loading the next round into the firing chamber. Usually the recoil spring (also known as main spring) tension pushes bolt back into battery and a cam strips the new round from a feeding device, belt or box.
Cycle is repeated as long as the trigger is activated by operator. Releasing the trigger resets the trigger mechanism by engaging a sear so the weapon stops firing with bolt carrier fully at the rear.
The operation is basically the same for all semi automatic or automatic weapons, regardless of the means of activating these mechanisms. Some examples:
- Machine pistols and submachine guns (like the World War II "grease gun", MAC-10 or the Uzi) are usually blowback operated.
- Most assault rifles and squad automatic weapons are gas operated. Some weapons, such as the AR-15/M16, do not have a piston, instead using a system of direct impingement in which the gases operate the bolt carrier by acting directly on it. Others, like the original SA80 patterns, have a bolt carrier that is unlocked and operated by a piston actuated by gases.
- A recoil actuated machine gun uses the recoil to first unlock and then operate the action. Heavy machine guns, such as the M2 .50 and Browning .50, are of this type. A cam, lever or actuator demultiplicates the energy of the recoil to operate the bolt.
- An externally actuated machine gun uses an external 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 Gatling guns in reference to their driving mechanism. Gatling guns have several barrels 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. Rotary 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, but this is not possible if the force needed to eject the round comes from the round itself. Rotary guns are generally used with large shells, 20 mm in diameter or more, offering benefits of reliability and firepower, though the weight and size of the power source and driving mechanism makes them impractical for use outside of a vehicle or aircraft mount.
- Revolver cannon were developed in World War II by the Germans as the Mauser MK 213 to provide high-caliber cannon with a reasonable rate of fire and reliability. A recoil-operated carriage holds a revolving chamber with typically five chambers. As each round is fired, electrically, the carriage moves back rotating the chamber which also ejects the spent case, indexes the next live round to be fired with the barrel and loads the next round into the chamber. The action is very similar to that of the revolver pistols common in the 19th and 20th centuries, giving this type of weapon its name.
Firing a machine gun produces great amounts of heat. In a worst-case scenario this may cause a cartridge to fire even when the trigger is not pulled, potentially leading to damage or causing the gun to cycle its action and keep firing until it has exhausted its ammunition supply or jammed. To prevent this, some kind of cooling system is required. Early heavy machine guns were often water-cooled; while very effective, the water also added considerable weight to an already bulky design. Air-cooled machine guns feature quick-change barrels, often carried by a crew member. 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. Some designs - such as the many variants of the MG42 - are capable of rates of fire in excess of 1500 rounds per minute.
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. 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.
It would not be until the mid-19th century that successful machine-gun designs came into existence. The 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. Dr. Gatling also experimented with electric-motor-powered models; this externally powered machine reloading has seen use in modern weapons as well. The Vandenburg and Miltrailleuse volley (organ) gun concepts have been revived partially in the early 21st century in the form of electronically controlled, multibarreled volley guns. It is important to note that what exactly constitutes a machine gun, and whether volley guns are a type of machine gun, and to what extent some earlier types of devices are considered 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 
The 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. (25.4 mm) caliber, flintlock revolver 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 1777, Philadelphia gunsmith Joseph Belton offered the Continental Congress a "new improved gun", which was capable of firing up to twenty shots in five seconds, automatically, and was capable of being loaded by a cartridge. Congress requested that Belton modify 100 flintlock muskets to fire eight shots in this manner, but rescinded the order when Belton's price proved too high.
In the early and mid-19th century, a number of rapid-firing weapons appeared which offered multi-shot fire, and a number of semi-automatic weapons as well as volley guns. Volley guns (such as the Mitrailleuse) and double barreled pistols relied on duplicating all parts of the 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 semiautomatic.
The Agar Gun, otherwise known as a "coffee-mill gun" because of its resemblance to a coffee mill, was invented by Wilson Agar at the beginning of the US Civil War. The weapon featured automatic loading through ammunition being loaded in a hopper above the weapon. The weapon featured a single barrel and fired through the turning of a hand crank. The weapon was demonstrated to President Lincoln in 1861. He was so impressed with the weapon that he purchased 10 on the spot for $1,500 apiece. The Union Army eventually purchased a total of 54 of the weapons. However, due to antiquated views of the Ordnance Department the weapons, like its more famous counterpart the Gatling Gun, saw only limited use.
The Gatling gun, patented in 1861 by Richard Jordan Gatling, was the first to offer controlled, sequential 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; it was subsequently improved and used in the Franco-Prussian war and North-West Rebellion. Many were sold to other armies in the late 19th century and continued to be used into the early 20th century, until they were gradually supplanted by Maxim guns. Early multi-barrel guns were approximately the size and weight of contemporary artillery pieces, and were often perceived as a replacement for cannon firing grapeshot or canister shot. The large wheels required to move these guns around required a high firing position which increased the vulnerability of their crews. Sustained firing of gunpowder cartridges generated a cloud of smoke making concealment impossible until smokeless powder became available in the late 19th century. Gatling guns were targeted by artillery they could not reach and their crews were targeted by snipers they could not see. The Gatling gun was used most successfully to expand European colonial empires by killing warriors of non-industrialized societies.
The Gatlings were the first widely used rapid-fire guns and, due to their multiple barrels, could offer more sustained fire than the first generation of air-cooled, recoil-operated machine guns. The weight, complexity, and resulting cost of the multibarrel design meant recoil-operated weapons, which could be made lighter and cheaper, would supplant them. Recoil-operated machine guns were light enough to be moved by one man, were easier to move through rough terrain, and could be fired from a lower, protected position. It would be another 50 years before the concept was again used to allow extremely high rates of fire, such as in miniguns, and automatic aircraft cannon.
The first 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's other great innovation was 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, most famously - during stalemate at The Battle of the Somme. The design required fewer crew, was lighter, and more usable than earlier Nordenfelt guns and Gatling guns.
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 MP18) as well as lighter machine guns (the Chauchat, for example) saw their first major use in World War I, along with heavy use of large-caliber machine guns. The biggest single cause of casualties in World War I 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 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 or having a pusher propeller.
Interwar era and World War II 
During the interwar years, many new designs were developed, such as the Browning M2 and the Thompson sub-machine gun, which, along with others, were used in World War II. The trend toward automatic rifles, light machine guns, and more powerful sub-machine guns resulted in a wide variety of firearms that combined characteristics of ordinary rifles and machine guns. The Cei-Rigotti (20th century), Fedorov Avtomat (1910s), AVS-36 Simonov (1930s), MP44, M2 Carbine, AK-47, and AR-15 have come to be known as assault rifles (after the German term sturmgewehr). Many aircraft were equipped with machine cannon, and similar cannon (nicknamed "Pom-pom guns") were used as antiaircraft weapons. The designs of Bofors of Sweden and Oerlikon of Switzerland were widely used by both sides and have greatly influenced similar weapons developed since then.
Germany developed during the interwar years the first widely used and successful general-purpose machine gun, the Maschinengewehr 34. The Maschinengewehr 42 was developed from it and was much cheaper to produce. The current GPMG of the German Army, the MG3, is a direct evolution of the MG42. Many other modern machine guns, including the US M60 and the FN MAG borrow elements of the design of the MG42.
Conventional machine-gun development has been slowed by the fact that existing machine-gun designs are adequate for most purposes, although significant developments are taking place with regard to antiarmor and antimissile weapons.
Electronically controlled machine guns with ultrahigh 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 probably continue. Another example is the six barreled, 4000 round per minute, XM214 "six pack" developed by General Electric. It has a complex power train and weighs 85 pounds, factors which may, in some circumstances, militate against its deployment.
Metal Storm has developed a new type of machine gun, with rates of fire up to 1.62 thousand rounds per minute. The distinguishing features of this technology are the absence of ammunition feed and casing ejection systems (the only moving parts are the projectiles), and the electronic ignition of the propellant charges.
Human interface 
The most common interface on machine guns is a pistol grip and trigger. On earlier manual machine guns, the most common type was a hand crank. On externally powered machine guns, such as miniguns, an electronic button or trigger on a joystick is commonly used. Light machine guns often have a butt stock attached, while vehicle and tripod mounted machine guns usually have spade grips. In the late 20th century, scopes and other complex optics became more common as opposed to the more basic iron sights.
Loading systems in early manual machine guns were often from a hopper of loose (un-linked) cartridges. Manual-operated volley guns usually had to be reloaded manually all at once (each barrel reloaded by hand). With hoppers, the rounds could often be added while the weapon was firing. This gradually changed to belt-fed types. Belts were either held in the open by the person, or in a bag or box. Some modern vehicle machine guns used linkless feed systems however.
Modern machine guns are commonly mounted in one of four ways. The first is a bipod – often these are integrated with the weapon. This is common on light machine guns and some medium machine guns. Another is a tripod, where the person holding it does not form a 'leg' of support. Medium and heavy machine guns usually use tripods. On ships and aircraft machine guns are usually mounted on a pintle mount – basically a steel post that is connected to the frame. Tripod and pintle mounts are usually used with spade grips. The last major mounting type is one that is disconnected from humans, as part of an armament system, such as a tank coaxial or part of aircraft's armament. These are usually electrically fired and have complex sighting systems. For examples of this, see US Helicopter Armament Subsystems.
See also 
- Marchant-Smith, C.J., & Haslam, P.R., Small Arms & Cannons, Brassey's Battlefield Weapons Systems & Technology, Volume V, Brassey's Publishers, London, 1982, p.169
- In United States law, a Machine Gun is defined (in part) by The National Firearms Act of 1934, as "... any weapon which shoots ... automatically more than one shot, without manual reloading, by a single function of the trigger."
- U.S. Army Training and Doctrine Command Field Manual 3-22.68 "Crew-Served Machine Guns", para. 4-207 https://rdl.train.army.mil/soldierPortal/atia/adlsc/view/public/6713-1/fm/3-22.68/chap4.htm#sec5
- Henderson, Charles. Marine Sniper Berkley Caliber. (2005) ISBN 0-425-10355-2.
- Harold L. Peterson (2000). Arms and Armor in Colonial America, 1526-1783. Courier Dover Publications. pp. 217–218. ISBN 0-486-41244-X.
- United States Continental Congress (1907). Journals of the Continental Congress. USGPO., pages 324, 361
- Emmott, N.W. "The Devil's Watering Pot" United States Naval Institute Proceedings September 1972 p.70
- Emmott, N.W. "The Devil's Watering Pot" United States Naval Institute Proceedings September 1972 pp.72
|Wikimedia Commons has media related to: Machine guns|
- GunTrustLawyer.com – US site with information on the legality of owning a machine gun in each state under the National Firearms Act and individual state regulations.
- How Stuff Works – Article on the operation of Machine Guns, animated diagrams are included.
- U.S. Patent 15,315 – A patent for an early automatic cannon
- Vickers machine gun site
- The REME Museum of Technology – machine guns
- Discover Military Machine Guns
- From Gatling to Browning September 1945 article Popular Science