I've been under the notion that a proper .223/5.56 load out of a carbine actually has less chance of overpenetration than a handgun or buckshot load. Which is one of the reasons many LEO agencies have been switching out their mp5 sub guns for SBR'd ar-15s. The theory is a smaller, higher velocity round will break away, deflect, or expand much faster than a heavier, slower round with more inertia behind it. Maybe I'm wrong... but I'm under the assumption that any round capable of providing adequate penetration to a perp is going to over penterate a normally constructed wall.
Of course, non of this matters if you don't know the shooting lanes in your residence. In my 2 bedroom corner apt, I know which way I'll be shooting into cement, and which way I'll be aiming into a neighbors place. As long as the intruder isn't using my toilet, or standing directly in my doorframe to the outside, I have a pretty good idea where my rounds will end up.
I started off using a shotgun for HD. I didn't have a tac light for it, so I did the next best thing and ziptied an old led to the magazine tube. Figuring that if I ever did have to fire it, the light would be toast, but I would at least know where that shot would be going. After I bought my .45, I practice clearing my house with a flashlight "LEO" style. I found it much easier to manipulate light switches and doors with my handgun that I could with my 20" shotgun. After I bought my first AR-15, I practiced clearing my house with it, and found it handled better than my shotgun, and was much more "on point" than my handgun. But I wont use it as my go to HD gun until I get a (for real this time) light set up for it. Til then I'm still using my handgun.
I just hope everyone is keeping a dedicated flashlight either attached to, or living next to their primary HD gun.