If the shoulder has deformed, the use of a shim may be appropriate. If it is the threads which have loosened, the technique is to set up the threads by using a radiused punch to move the threads toward the shoulder. Lightly tapping around the threads will do the trick. This is the traditional method. An alternative is to make a threaded sleeve of 4140 ( a vee thread of the same pitch as the square thread on the barrel) which is then threaded on, almost to the shoulder. You want to space the sleeve away from the shoulder about the amount you want to move the top of the thread. Then, with the barrel held in the barrel vise, hit the sleeve a pretty good shot, square on, with a hammer. You don't want to over-do it so hit and try. The vee thread will bend the square thread forward rather than starting a sheer at the root. Either way works but the first method is quicker while requiring a deft touch. The second method may be a little more precise. Regards, Bill