We are alike OP.
I mained in shotguns to being with, mainly the S12, loved it. i was doing good for the most part with it. I started using other guns like assaults and snipers and stuff...but like you too many people used SMG's and i didn't want to cave in...so i stayed away as long as possible, until i got owned after owned after owned by them as i progressed in multiplayer.
So i tried the MSMC...never went back unfortunately. it's too good. quick movements, hardly any recoil and easily controllable at range in burst taps, kills super fast, has accuracy and firing forgiveness...it just can't be beat..SMG's in general just can't be beat.
I seriously just got out of games with it doing 60+ kills to 2-5 deaths because it allows me to kill so incredibly fast especially flanking with a silencer you're practically untouchable i swear!
any other gun i just cannot perform like this, maybe the rare time with other guns but not nearly as much as with the SMG classes...
maybe it is due to the map designs...maybe it is the guns ease of use and forgiveness when it comes to recoil/aiming/firing...but imo it's OP'd as hell.
like i have diamonds on shotguns, and the KSG was super fun to use...but realistically who would use that over a SMG? you risk too much, you have to aim with perfect precision, you also have to have pin point timing with the fire button or you're in for a pump, and there's just no forgiveness at all...yes its a one shot at a decent range and close range, but SMG's kill practically as fast and you can miss and miss-fire and still come out the victor just due to the fact that spraying doesn't give enough recoil.
in all honesty, massive recoil would solve a lot of issues, people wouldn't be able to just hold down R1, they'd have to master their weapons firing patterns to be boss with it, cuz right now it's a battle to see who can spray better...of course pro's will disagree with me, but i'm averaging an almost 2.0 KDR and i can say that there is no other easier gun to use than the SMG AND be HIGHLY outrageously effective with it...
I actually get killed by and use all sorts of weapons in this game. Hell, I've been killed by just as many Type 25s and AN-94's lately, does that mean they need a nerf too? Get over it dude. People run and gun in this game, and for that, they SMGs are boss. You want to use LMGs and ARs more effectively, tell Treyarch that you'd like to see larger maps with long lines so that there are maps that are leaned more towards ARs and LMGs, and not so much Shotguns and SMGs.
So your whole frustration is based on too many people in your opinion use the same group of guns. Well, I'm afraid that you are going to continue to be frustrated. They could take the SMGs out of the game completely and then some other handful of guns would be all that you would see. Nerfing SMGs into oblivion won't help. Now personally, I do see a lot of SMGs out there but I also see a lot of ARs, Snipers, Shotguns, and the least amount would be LMGs. If anything I'd say that BO2 already has it's predecessor BO1 beat for gun variety usage (seriously AK74u, FAMAS, AK47, Commando and the occasional AUG). However, to achieve this variety they had to make each class a viable option. So you get a ton of OP threads by everyone that's not used to having competition from something other than their class of choice. No one class dominates in BO2 and because of this you actually have to prepare your gameplay to take into account that yes an SMG can kill you, yes a Shotgun can kill you, yes an AR can kill you, and yes an LMG can kill you and what's more you are likely to have to defend against all of them on any given map. It's a more difficult game than what the community is used to. So put in the work to learn how to handle this new dynamic and then you won't care so much about what guns other people use.