I've been doing Lightweight/Hardline SMG classes since BO1, and I love them so much. Also, I prefer the Chicom or Skorpion as my SMG in this game because the MSMC I consider OP and too popular, and I like being set apart. (I think those that use the MSMC have magical lag-manipulating powers to make you lose to them 100% as long as you are using an SMG).
Simple answer to the msmc... I've used it a lot, but the fact is that I miss a lot and the msmc fire rate is too low
Try the Skorpion. I switch between a Skorpion, a Rapid Fire LMG, and a Dual Band Sniper Rifle depending on the map/objective. I've found that I tend to do best with the Skorpion, due to its high rate of fire. I do well with the MP7, too, but when it comes to twitchy reflex shooting, I win almost every time with the Skorpion.
I play on Hardcore, though, so that probably makes a huge difference.