They put shotguns in the "heavy" slot because it is usually a one shot kill. You should try to be more strategic when using the power slot no matter what weapon type it is. The power slot is designed to level the playing field and maybe get on a streak, shotguns ran rampant in D1 and ruined the point of gun play. Don't rely on the shotgun anymore because it's never going to change, it's a power weapon and if you want to get kills with it, it will happen if you play with strategy and grab some power ammo.