Machine gun A machine gun is a fully automatic mounted or portable firearm , usually designed to fire bullet s 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 gun s , machine guns , or autocannon s . 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 firearm s , 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 weapon s ( 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 . " The majority of machine guns are belt - fed , although some light machine guns are fed from drum or box magazines , and some vehicle - mounted machine guns are hopper - fed . 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 pistol s 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 rifle s or battle rifle s , while rifles that fire an intermediate cartridge ( see below ) are called assault rifle s . 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 rifle s 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 weapon s " 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 gun s , 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 . Operation . All machine guns follow a cycle : 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 : 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 . History . 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 gun s . 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 sniper s 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 minigun s , 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 gun s 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 gun s ( 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 entanglement s , machine guns earned a fearsome reputation . The automatic mechanisms of machine guns were applied to handguns , giving rise to automatic pistols ( and eventually machine pistol s ) 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 gun s , 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 rifle s ( 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 . Future . 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 million 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 gun s 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 . References . Notes