Basic Information
The General Purpose Machine Gun is a weapon that can fulfill the roles of both the Light Machine Gun and the Medium Machine Gun. This is generally a rifle calibre weapon, belt fed with all the characteristics of a light machine gun which can also accept further components - such as a tripod mount and improved sights - to allow it to function in a sustained fire mode.
Almost all of these weapons are now belt fed and fitted with quick-change barrels for improved fire volume and ideally these will be lavishly supplied when the gun is set up in sustained fire mode - in light mode the supply is usually limited to what can be carried by a single man, plus any extra capacity that can be found in his team or section. Most militaries compromise by having the gun deployed in light role, and then providing additional stores for the sustained fire role to be either stored on a unit's organic transport or carried by other members of the gun group.
The first weapon to qualify as a GPMG was the WW2 era MG34/42 and its fingerprints are found to some degree in all subsequent weapons - notable examples would be the FN-MAG (of which the M240 and the L7 are little more than re-labellings), the M60, the MG3 (more of a re-incarnation than a descendent) and the PKM.
Most national armouries have a GPMG as their standard machine gun and these weapons, or simple variants thereof, will be found as infantry weapons, main and auxillary armaments on land vehicles, flexible mount weapons on aircraft, small boat guns at sea and, not infrequently, fortress guns in places that still use them.