so im sure this has been asked before but i have never paid that close of attention to the topic. i am interested in a upper that a shop has thats 11 inch i think. so can i take my lower from another rifle that is a 16 incher and do the proper paper work with the atf and get the tax stamp from that lower. or do i need to buy a pistol lower or a lower that has never been made into a gun at all. thanks for any help with this. also if i built a pistol first can i change that into a sbr later after i get the tax stamp.