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..................
do you know if the specialty rounds are 9mm or 5.56 standard round for the AR? I'd love to have an AR myself and if I did it'd want the 5.56 as thats what i'm use to with being in the army. But if those rounds are 9mm only. That might make that version a little more interesting.
As far as I know they come in 5.56 and 7.62 platform. The link below will give you all the info on their Hornady TAP line and the rifle rounds I'm talking about are the 40 grain Hornady TAP urban rounds. On page 52 of this link it shows you the x-rays of the bad guy who took one to the head from a 5.56 round and how that round stayed in his head and didn't go any further. Now this is a law enforcement and military line, but I'm sure just like anything they either offer a civilian version or it's possible you could get your hands on some, but I don't know that for sure. I kinda want some of our patrol officers to go to the urban rounds because of how they expand their energy so fast, which you can see in the ballistic testing in that link, so there is really no chance of a through and through and it hitting something else. There's always a chance of that, but it would be a lot smaller than with a 55 grain 5.56 federal round that is your "normal" 5.56 round as far as ballistics go. Sorry to nerd out on the ballistic stuff, but for some reason I find that really interesting. I'd rather know more about ballistics than all the different weapons out there. So for Hornady to come out with a round like this is impressive IMO. I took part in ballistic testing here in Iowa with them at our law enforcement academy. Really impressive stuff, but I couldn't get our Chief to agree to buy the stuff because it was a tad bit more expensive than what we currently use. The stuff we currently use is great stuff, but Hornady's line retained its weight better during testing which is huge.
http://www.hornadyle.com/assets/site/files/hornady_tap_report.pdf