<P> An externally actuated weapon 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 or chain guns in reference to their driving mechanism . Gatling guns have several barrels each with an associated action on a rotating carousel and a system of cams that load, cock, and fire each mechanism progressively as it rotates through the sequence; essentially each barrel is a separate bolt - action rifle using a common feed source . 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 in the rare cases of self - powered rotary guns . Rotary guns are generally used with large rounds, 20mm 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 . </P> <P> Revolver cannons, such as the Mauser MK 213, were developed in World War II by the Germans to provide high - caliber cannons 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 . </P> <P> Firing a machine gun for prolonged periods produces large amounts of heat . In a worst - case scenario this may cause a cartridge to overheat and detonate 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 (this is known as cook - off, distinct from runaway fire where the sear fails to disengage when the trigger is released). To prevent this, some kind of cooling system is required . Early machine guns were often water - cooled; while very effective, the water also added considerable weight to an already bulky design . Air - cooled machine guns often feature quick - change barrels, often carried by a crew member, passive cooling fins, or in some designs systems of forced - air cooling such as that employed by the Lewis Gun . Advances in metallurgy and use of special composites in barrel liners allow for greater heat absorption and dissipation during firing . 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 1,200 rounds per minute . </P> <P> 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 machine guns have a "safety" sear, which simply keeps the trigger from engaging . </P>

When was the first machine gun used in world war 1