My primary HD weapon at the moment is a stainless .223 mini-14 with a Choate pistol-grip stock; ammo is Federal 40-gr JHP in a 30-rd magazine (downloaded to about 24 rounds), so you know which one I'll recommend!
The 40-grainers are extremely unlikely to penetrate an exterior wall, the pistol grip stock aids in retention and maneuvering in close quarters, and at home-defense ranges I keep the rear sight folded down and aim with the front sight only, if needed.
I'd recommend the -14 over the -30 for reasons of capacity, wider availability of full-capacity magazines, and much lower likelihood of overpenetration with the right ammunition.
TaurusGL is right, a short-barreled shotgun is also a good choice, but I prefer the rifle for its ability to make a precision shot if needed, greater magazine capacity, and ergonomic reasons. I will say, though, that across-the-room distances, a shotgun pattern will only be a couple inches wide, so you will still need to aim (just a little less precisely than with a rifle or handgun).