I don't know, but if I worked for the BATFE I think I'd try to locate some authority on whether a "short-barreled rifle" has to be shoulder-fired. How do they treat a short-barreled shotgun with only a pistol grip that can be easily-modified to fire from the shoulder? I'm thinking about the old U.S. v. Miller case (1939) that's still on the books after Heller and McDonald. AOW, in my view, would be too nebulous for a lot of U.S. Attorneys' offices to get their arms around at the courthouse.I have built several full size AR rifles, and am now trying my hand with a pistol version. I have the lower done, and starting on the upper. My question is whether I can put a forward grip on a 7.5" or 10" barrel pistol. I think I have heard or read somehwere that it is forbidden, but not sure, and can't imagine why that would be. Any help?