from the research i have done a pistol tube is not a requirement. Bunch of fellas use a carbine tube and wrap it in 550 cord. The only issue I see is the constructive intent one. If you have a spare stock lying around it could easily be attached and you have your constructive intent. Cant do that with a pistol tube so it give you that safety margin. Thats just what I've seen, but not a lawyer and never been involved with anyone that's been affected by this besides being berated by other non ATF folks.