You're welcome Rolf. The best I have found, aside from a lathe, is to open up the breech end with an adjustable hand reamer. It can be a slow process but a high level of precision is possible because you have total control. Starting the threads with a taper tap will minimize the likelihood of the threads ending up crooked. A small level, provided the barrel is level, can be used to verify if the tap is starting correctly. After cutting a few threads with the taper tap the remainder should be cut using a bottoming tap. This allows a clean and tight breech plug to barrel bore fit.
Hank
