Here is a good setup:
MSMC
-Silencer, fast mag.
-Perk 1 ghost, Lightweight
Perk 2 toughness, scavenger
Perk 3 Dexterity.
This gun has long range and moderate recoil meaning grip and long barrel are waste of attachments.
It's reload time is really slow so fast mag is the best option. I choose silencer just to not attract attention.
Lightweight makes you super fast. Scavenger just never run out of ammo. Dexterity climb faster and your gun will be readied faster. Ghost so you won't appear on radar when the enemy has UAV. Toughness, no flinch well there is flinch but it's less. Score streaks UAV, CUAV, Orbital VSAT/AGR.
Use CUAV only if the enemy manages to get Orbital VSAT. :D