Not enough umph behind the weight. Trying to drive clean through a strip mall with a semi going 5mph you can't expect to hit that damn hollister in the middle. Point being, big rifles were made for a reason. Slugs only make shotguns slightly more versatile, but a 650 grain slug being pushed by what you can load into a 12 gauge casing is really gonna have trouble making it into big and dangerous games "boiler rooms".
