Machine guns fire rifle calibre cartridges and will fire a stream of bullets with one pressure on the trigger or firing device. After each cartridge is fired, the weapon reloads itself and if the trigger is still depressed, the weapon will fire that next round, and continue this cycle until the firing mechanism (e.g. trigger) is released, it runs out of ammunition or the weapon jams. This is limited of course by the method of feeding the ammunition, e.g. by belt or by magazine. As well, the weapons would heat up and how long it could keep shooting depended upon the cooling method.
The Gatling Gun is a predecessor of the machine gun, but because it uses a hand-crank to fire, it is not automatic. It was invented in 1861 by Dr. Richard J. Gatling to reduce the size of armies and so reduce the number of deaths by combat and disease, and to show how futile war is. In 1884, Sir Hiram Maxim figured out how to make the weapons reload itself, thus inventing the machine gun.
Machine guns are generally grouped into the following categories. Sub-machine Guns are considered to be a different category.
- Light Machine Guns (LMG) – These usually have a bi-pod (two-legged folding stand) and can be carried by one soldier. These usually have a box magazine. Examples include the Lewis Gun, Czech ZB-26, British Bren Gun(developed from the Czech LMG), French Chauchat, and the American BAR M1918.
- Medium Machine Guns (MMG) – These are usually fired while mounted on a tripod (3-legged stand). Well known examples are the Vickers Mark I and the Maxim M.G. 08. These required a team of soldiers to move and operate.
- General Purpose Machine Gun (GPMG) – These can be operated either on a bi-pod or a tripod and have a sustained rate of fire capability. Examples are the German M.G. 34, M.G. 42 and American M60 (derived from the German MGs). Although the Bren Gun had a tripod option, and even a 100 round drum magazine, it is not considered to be a GPMG. The Canadian Armed Forces mistakenly called their 7.62 mm version of the Browning M1919A4 a GPMG. A bi-pod did exist for the M1919, in the M1919A6 version complete with butt-stock, but Canada did not use these. It was only used as a medium machine gun, but called was incorrectly called a General Purpose Machine Gun (GPMG) as it did not have a bipod except on the M1919A6 variant.
- Heavy Machine Guns (HMG) – These tend to be larger and the best known example is the .50 calibre Browning M2 machine gun. They are usually mounted on a heavy tripod, a vehicle or an aircraft.