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The first true machine gun was invented in 1881 by 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. 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. The design required less crew, was lighter, and more usable than earlier 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. Submachine guns (e.g., the Thompson, or 'Tommy gun') as well as lighter machine guns (the BAR, 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 WW1 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.

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A machine gun is a fully-automatic mounted or portable firearm, usually designed to fire rifle cartridges in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred bullets per minute. Earlier machine guns were manually operated, for example, by turning a hand crank. In US law, the term machine gun is used non-specifically to describe any automatic firearm, for example in news media or legal code.

There are two main different definitions of the upper limit of caliber for machine guns: 1) caliber larger than 12.7 mm (.50 caliber); or 2) larger than 20 mm. Larger-caliber automatic weapons are generally referred to as autocannons. In-between, there are weapons that have been called by either name depending on other traits; for instance, there have been weapons of roughly 15 mm that were variably referred to as autocannons and machine guns.

Another factor is whether the weapon fires conventional rounds or explosive rounds. Automatic weapons firing large-caliber explosive rounds are generally either autocannons or automatic grenade launchers ("grenade machine guns"). Machine guns tend to share a very high ratio of caliber to barrel length (a long barrel for a small caliber).

There have been two main machine gun eras: the era of manual machine guns and the era of automatic machine guns. The technical development itself is marked by a series of developments of specific automatic features, as well as technical developments (such as linked ammunition). The era of manual multi-shot devices extends back hundreds of years (such as manual volley guns), but the development of manual and automatic machine guns takes place almost entirely in the latter half of the 1800s. Manual machine guns are manually-powered, e.g., a crank must be turned to power reloading and firing, as opposed to simply holding down a trigger, as with automatic machine guns. There are many other notable features, but this is one of the most significant to allowing higher rates of fire common to machine guns.

Manual machine guns, as well as manual volley guns, saw their first major use in the American Civil War. The Gatling gun and "coffee gun" both used manually-powered automatic loading, fed via a hopper filled with cartridges. The Gatling gun ? a manually-powered rotary machine gun ? would be the major type of the late 19th century, though there were many other manual designs with varying degrees of use (e.g. the Nordenfelt machine gun). The first automatic machine gun was the recoil-operated Maxim gun, which used linked (belt) ammunition, as well as a single barrel and automatic loading. This concept of using bullet energy would also drive the development of nearly all other semi and fully automatic firearms of 20th century.

The two major operation systems of modern automatic machine guns are gas operation (which uses the gas generated from the burning powder to cycle the action), or recoil operation (which uses the recoil generated from the ejecting bullet to cycle the action). The first gas-operated machine gun was the Colt-Browning M1895. Another (minor) type is the externally-powered machine gun. Rather than human manual power or bullet energy, a third source (such as an electric motor) is used; these types are now called by more specific names (see Minigun, Chaingun). The most common type of modern machine gun remains the automatic, recoil-operated and belt-fed type. Electrical and Gatling-type machine guns are common on fighting aircraft and other vehicles.