Currently, stripped AR lowers are marked as receivers on Form 4473 (Section D, box 29). You can build it into EITHER a rifle or a pistol. Before the current 4473, I think they were marked as "other," but that shouldn't affect anything.
I am not aware of any AR lowers that are marked "pistol," but even if some are, that doesn't change the fact that until you build it into something, it isn't anything but a receiver. I believe you can built it into a pistol or rifle as you choose.
It wasn't that long ago they issued a ruling about whether something built as a pistol could later be swapped into a rifle (or vice versa), but heck if I can remember what it was.
Last edited by Technosavant; December 26, 2012 at 10:32 PM.