Except, in Core, I need to keep a target down my sights with a semi automatic weapon that actually has noticeable recoil in this game and that has pretty marginal aim assist for 3-4 shots as the target shoots at me. When I use a shotgun at this range, I can quickly scope in, fire once, and since most shotguns have some form of delay in firing regardless of auto fire/semi auto, I can scope in and fire again if necessary, more controlled, having an easier time strafing, having more wiggle room because of shotgun spread, and killing them in 1-2 shots.
In hardcore, I see a target, I aim down my sights, and kill it in one shot, shotgun or pistol. Except in this case, I can beat people at range with my pistol and can't with my shotgun. Since I can't absorb a shot or two to get around a corner if I'm caught in a bad position like I can in Core, a pistol is superior.
In most hardcore game modes I replace any case I would use a Shotgun class with a pistol class for the lulz.
As for the SMG debate, I prefer the MP5 to the Super Soaker. Call me crazy. I love my MP5. <3