So if you watched the video, you can see that I have a problem with the big thread between the breech and the barrel, at least in the wooden prototype. It might work better in brass or gunmetal.
The thread has a pitch of 6mm and a diameter of 60mm. It is big.
My plan at this time, is to make a brass male threaded section, and glue or screw it into the breech. Then to make a steel tap using the same G code, and cut a thread into the wood of the barrel. (p.s. note 30 Sep… I continued to experiment with feeds, speeds, and cutter shapes in the wood. The final result was OK so I did not make metal threads. That will have to wait until I do this project entirely in gunmetal or brass… maybe never)