Hairball,
Although a lot of people might disagree with me, I don't really see the point of an AR shooting a pistol caliber, except for just the fun factor. In my opinion, if you are going to shoot a pistol caliber, shoot a pistol. The pistol rounds do not pick up that much more velocity in a carbine length barrel vs. a full size pistol. I feel that if you are going to be carrying/using an AR and you have to deal with the weight and size of an AR, why not have it in a superior caliber, i.e. .223 vs. any pistol caliber? Just my opinions, of course...