If I want to take my tranferable M16A1 and put a 11.5 inch upper on it do I have to pay for another tax stamp and SBR it. If I keep it that way and travel with it, is there anything I have to do or tell the ATF. If i dont want to keep it that way but just want to go back and forth do i have to do anything? And in the future if I sell it do I have to put it back as a 20 rifle again before I can sell it? I just want to learn the right way of doing things. Thank you, Semper Fi