While early light machine guns fired full calibre service ammunition, modern light machine guns often fire smaller-calibre cartridges than medium machine guns – generally the same intermediate cartridge fired by a service's standard assault rifle – and are usually lighter and more compact. Some LMGs, such as the Russian RPK, are modifications of existing designs and designed to share the same ammunition. Adaptations to the original rifle generally include a larger magazine, a heavier barrel to resist overheating, a more robust mechanism to support sustained fire and a bipod.
A light machine gun is also defined by its usage as well as its specifications: some machine guns – notably general-purpose machine guns – may be deployed either as a light machine gun or a medium machine gun. Deployed on a tripod and used for sustained fire, it is a medium machine gun; if deployed with a bipod with the operator in a prone position and firing short bursts, it is a light machine gun.
Light machine guns are also designed to be fired from the hip or on the move as a form of suppressive fire intended to pin down the enemy. Marching fire is a specific tactic that relies on this capability.
Lighter modern LMGs have enabled them to be issued down at the fireteam level, with two or three at the section/squad level.