The breach nut is just a round nut threaded 20 tpi that screws onto the threaded end of the barrel to hold it against a sholder at the front of the stub, the rear of the stub is recessed about a half inch in the diameter of the nut. Most of the time I use a turned piece of the shotgun barrel to make the nut with. Most of the barrels use a barrel band for the forearm stud to attach to. On one project it worked out so that I turned the inside of the shotgun barrel with the stud attached to slide over the barrel and put a set screw through the forearm mount and locked it to the barrel that way. This project may have enough barrel diameter to drill and tap a stud directly into the barrel. Nothing secret here, I will tell you anything I know. I fill the .002 gap between the stub and the barrel with JB weld as a bedding agent, this leaves the entire stub with no voids in it, so it probably gains a little strength from the stub part of the barrel also. The pin is just an extra holding point, I drill it through the stub and barrel in front of the hinge point, you can see the end of the pin in the picture. Larry