As I recall I used the diameter of the front of the 12 gauge chamber for the major diameter of the threads, cut them 20TPI so I didn't have to cut them too deep. For a high pressure round I think I would bore the 12 gauge barrel chamber out a little to make the breach end of the barrel a little thicker than the one I used for the 30 carbine project. Do the work on the stub first, then if you hose it up you can make the barrel match the stub, remember, you don't have to stick to any standard diameter and thread pitch. The front part of the 12 gauge barrel makes a great barrel nut. Larry