Keep in mind that most screw in choke tubes have a double start thread. You've got to somehow cut one start to the proper depth then do the other one. It's not easy to go back and cut the first one more if you don't hit it right the first time. The correct way to do this is just break down and buy the proper tools - reamer, and tap with pilot that fits and just do it right. They cost some money, but if they keep you from ruining one barrel they've payed for themselves.