Well use this as a guideline,
all western forces (US UK NATO...) mostly use 5.56 ammo for their small arms, mgs such as m240, GPMG or the Mg3 use 7.62
the syrians or uncons also use 5.56 for most of their weapons exept the RPD, Dragunov, and the ak47 (or akm) (NOT THE AK 74 )