Most people would say SMGs are overpowered, but it isn't really the case. SMGs are short range weapons as a whole, so if you try to combat them at short range with a long range weapon you are obviously at a disadvantage. I have yet to find a weapon in this game (I only use AR and SMG, though) that is far and away better than any other. I usually arrive at a gun, think its the best, but after about a half hour I notice its deficiencies and move on.
I do hate the quickscoping with the sniper rifles, though, which is becoming more and more prevalent as time goes on. They need to nerf it, just make the draw time significantly longer. Just like you can't fast mag a LMG, you shouldn't be able to quick scope a sniper rifle. Snipers need time to set up IRL, and while COD is not like real life, it isn't an episode of family guy either.
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I tend to ask myself "Is this more balanced than commando knife lunging and One-Man-Army tubes in MW2?" If the answer is yes, it put it in the "pretty balanced" catagory. By that logic, btw, almost all of BO1 was well balanced.
This game, a little more issues, but still pretty good.
The maps play in favour of SMGs and Shotguns though especially the ones that seem to come up every other game like Hijacked, Raid etc.
IMO the 870 is a little unbalanced, the high damage isn't the problem but the random pellet spread. You should be rewarded for using a weapon effectively but right now you can OSK a guy a mile away (exaggeration FTW) but hitmarker a guy in you face. No gun should be based on Luck/Randomness.
Overall though I feel they did a decent job with weapon balance.