I mostly just lurk here these days but I'm compelled to say something about this:
If you were to choose a AR I would tell you to look at some specialty rounds for it from Hornady. The big complaint with AR rifle rounds is that, "They'll go on forever and wah wah wah..........." No they won't and there are speciality rounds out there now that expend all their energy into the target they hit. They won't go through the target and keep going. I have some sweet material on the ballistics of some of these new rounds. One example was a SWAT team making entry into a house, dude has a gun, SWAT guy puts one well placed round into the head of bad guy. The round never left the guys head, it stayed in there and expended all of its energy once it made contact. From a ballistic standpoint that's awesome..................
The FBI recommends a
minimum of 12" of penetration in ballistic gelatin for a reason. According to Hornady's own data the 40 gr Urban TAP rounds barely penetrates 6". That is a recipe for disaster for anyone going up against a determined assailent. Hell even Hornady points out that they are designed for a very limited application. Not only are you recommending it as a home defense round but you also want to issue it in patrol rifles? God forbid they ever encounter someone using cover.
FWIW the 40 gr Urban TAP is the 40 gr V-MAX with a different marketing pitch. I don't think Hornady even offers it any longer.
Also the x-rays on page 51 of the Hornady brochure actually show trauma caused by the 75 gr BTHP (Ballistic Tip Hollow Point) TAP, which
is a generally well regarded round.
Old myths aside, .223/5.56 is no more of a threat to penetrate of common building materials than handgun calibers commonly used for self defense. A couple of layers of drywall is not going to stop
any round that is capable of reliably stopping a human being so (as always) you need to be aware of what is beyond your target ("Threat, Backstop and Beyond") but to reiterate, 5.56 is no more of a threat in that regard than any other effective defensive caliber (including buckshot).