That last point is what I had always thought made shotgun a better option for a home defense situation -- less over penetration.

Assuming it's a stray shot, not one that already went through the threat, doesn't an AR-15 penetrate walls much more easily?

Where I live (the Soviet Euro Union), ironically, having a *less* deadly option is a real consideration. Shit is bad enough if you dare use your legal gun to defend your home (which is illegal in all cases where I live).