If you want a SBR (Short BArreled Rifle) it would be a waste to atart with a pistol lower. To have a stock and barrel less than 16" on any lower (pistol or rifle), you need to first register it with BATF on a Form 1 with two fingerprint cards and the making/transfer tax of $200. Once you receive the form back from BATF approved, you can add a stock to a pistol lower (with a short barreled upper) or a short barreled upper to a rifle lower. Here's the catch; you can remove the new NFA weapon from the NFRTR at any time by removing the short barrel from the rifle and writing to BATF to explain that it no longer is a short barreled rifle, but if you do the same with a pistol lower, it reverts to a rifle lower (not back to a pistol lower). A pistol, converted to a SBR, cannot be returned to a pistol configuration and removed from the registry because it [i]had[/i] a stock; it is now forever a rifle lower. Additionally, I believe even a preban pistol lower would have to be legally assembled as a postban SBR, though a preban rifle lower could be assembled as a preban SBR. Since the making of an SBR from a pistol lower would be creating a new rifle (ie. it was not a rifle prior to 9-13-94) I am fairly certain it would be treated as a postban SBR and so marked on the returned F1.