Use a harris bipod stud with rectangular nut. find position and use a punch for correct placement, drill hole (since you will be drilling with tube in place, slip something between tube and barrel for protection). Stud may have to be shortened, with some tricky manuveuring you can line nut up with hole drill (bent paperclip) and get stud started in threads and begin tightening with nail through stud. Once you start tightening, the inch or so long rectangular nut will not move and you can get the stud tight. Out of three freefloated uppers, I have done two this way, though on tube would clear the gas block for drilling. On two I used the harris bipod stud as I liked the extra area covered by the large nut. The stud is only about $10.00 though I have gotten other studs for $1.00 with little round nuts. DPMS told me to just drill a hole of a certain diameter and use the self tapping stud without a nut (if I remember right) but I like my way better though it is more difficult.