If you don't have Heavy Ammo+ then Field Scout + CQB Ballistics is nice. Persistence is a good perk as well. Not sure if Surplus adds ammo for sure on Heavy weapons in PvP but it does on Fusion Rifles so if it's the same then that'd give you the most ammo possible. Good roll. Keep unless you have Heavy Ammo+ and really want that extra bit of damage/stability/range from Aggressive Ballistics + Perfect Balance.