threads, and stay on the gun, even with mere 9x19 type pressures. The barrel walls on many guns are to thin to take a coarse thread. So you end up having to make a steel adapter. The ID of the adapter has the fine thread to match the barrel, the OD of the adapter has a coarse thread, to better hold in the aluminum rear cap of the silencer. An epoxy that can withstand some heat is used to lock the adapter into the rear of the can. You don't want any more wear of those threads, in the aluminum than the very minimum. The adapter should be at least "skin-hardened", with the torch and Kasenite powder,of course, and so should the threads on the barrel. This action will minimize the wear and stretching of the threads. It's an ESSENTIAL step for cans on which the front sight is mounted, for certain.