<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> <P> 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 . Dr. Gatling also experimented with electric - motor - powered models; this externally powered machine reloading has seen use in modern weapons as well . </P> <P> While technical use of the term "machine gun" has varied, the modern definition used by the Sporting Arms and Ammunition Manufacturers' Institute of America is "a fully automatic firearm that loads, fires and ejects continuously when the trigger is held to the rear until the ammunition is exhausted or pressure on the trigger is released ." This definition excludes most early manually operated repeating arms such as volley guns and the Gatling gun . </P>

When did the first machine gun come out