I my past experience with those who've used SBR's, basically an AR pistol with a stock, had very few benefits in using them in rifle calibers. Muzzle blast was horrendous, you couldn't be on either side of them within 15 feet and even behind them, on a covered firing line, was obnoxious, with hearing protection in both cases. With a suppressor at least it was tolerable to all involved, but once it got dirty and the rifle wouldn't function that turned into another story.
Lower muzzle velocity means less energy on target. This is real with the 223 in testing done with a myriad of barrel lengths. There's really not much going for it other than maybe the cool factor, IMHO. Having one in a pistol caliber makes more sense, to me, than any rifle caliber you can think of, maybe excepting the 300 BO.
So, as you may surmise, I'm not a fan of them.
Other than very limited storage space requirements I think it is what Jeff Cooper called an answer in search of a question.
