No need for balancing. Shotguns have only very few opportunities to one shot kill you. SMG's are far more lethal than shotguns. When you have a shotgun/SMG duel, then most of the time SMG's win.
I've tried shotguns multiple times and never have I ever gotten a one shot kill with a shotgun from a 20-30 meters away. Try it yourself in the training room and see how much damage the shotgun actually makes when the distance is that much. When the distance is less than 10 meters then you may have a chance to get one shot kill with it.