I have seen the 9MM carbine advocated in areas where handgun ownership is restricted. there was an article recently in American Rifleman by a retired SF E-9 on house defense and clearing with an AR-15. In a shotgun birdshot and skeet shot are not to be trifled with a close range. Wide spread, reduced penetration of walls, low recoil. And the shotgun-like a the submachine gun-requires a lot more training than most realize, the effectiveness of "point and pull" is "greatly exaggerated." What really counts is the combat mindset.
The guns are just tools. The mind is the weapon.