MG's will make better use of the heavy ammo drops. You don't blow your wad in 2-3 shots and can control a point as a single person with a full load, especially if you manage to take out the enemies with Heavy ammo before they get you. Best heavy for Crucible is Jolder's Hammer, IMO. Mine has Field scout so I have a 50 rnd belt and a huge impact. 2-3 rounds will kill so I can use minimal ammo and still have some left when the next heavy ammo spawn drops.