Modern Western MMG/GPMG weapons almost always fire 7.62x51 full power rifle ammunition; modern Eastern MMG/GPMG weapons usually fire 7.62x54R full power rifle ammunition with a rimmed cartridge.
Modern Western MMG/GPMG weapons almost always fire either the 5.56mm NATO round, or the 7.62mm NATO. Modern Eastern MMG/GPMG weapons usually fire 7.62x54R full power rifle ammunition with a rimmed cartridge.