The MSMC is a great SMG- my favorite by far based on damage and accuracy. I actually have developed the habit of minimizing the load out on it and use typically one or two of the following, in order of frequency:
Long Barrel
Fast Mag
Extended Clip
Silencer
Quick Draw (I ADS corners etc. so this doesn't help me much as the hipfire is fairly accurate for close snap engagements)
They all have their benefits but Long Barrel has the best add for my gameplay.
Current loadout:
MSMC + Long Barrel
Ghost + Flak Jacket
Scavenger
Tactical Mask
Executioner (prefer Tac-45, working challenges)
C4
Flash Bang (prefer concussion, working challenges)