If you want to make the effort, the way to get the best info is to call the Help line of one of the major producers of welding rod and ask, " Which (use vendor's name here) filler rod should I use to weld AISI 1137 to (whatever your breech plug would be made of)?" For example, if I were calling AIRCO, I would say. "Which Airco filler rod should I use to weld 1137 to 1018? Are any special techniques required?" Personally, I would only consider using TIG as the process, but that's me.