I'd say the Hornady 55 or 60 grain .223 TAP Urban would be a good choice. It's not designed to penetrate barriers and should still expand well out of an SBR. Hornady claims a muzzle velocity of 2650 out of a 10.5 inch rifle for the 55 grain. Whether it will penetrate walls I don't know. I've seen testing that says M193 will not penetrate walls because it tumbles, so the Federal 55 grain XM193, which is relatively cheap, might be worth considering although it's older technology. You definitely want something that expands and doesn't penetrate walls too much.
