The breech and the barrel are joined with a very large thread. On my 1:10 scale model it is 60mm diameter, and has a pitch of 6 mm. These dimensions are measured off Internet photos of the original bombard, so they might not be faithfully accurate to the original bombard. If anyone has accurate plans of the bombard I would be very interested to hear from them.
I experimented with various spindle speeds, feed rates, depth of cut, and finally decided that red gum wood is not the ideal material to be cutting a thread with sharp points. However, at 200rpm, and taking 50 cuts to reach the full depth, and using a very sharp tool, the end result was OK. I will fill the tearouts.
In order to make a functional join in the wooden cannon, I truncated the apex of the thread. In the gunmetal version I will attempt a more faithful to the original, sharp look.
For some reason, the wood held together better during the internal thread cutting than the external.
The male thread was cut on my newly CNC converted lathe, between centres, but the fixed steady on that lathe was just too small to hold the barrel, so the internal thread was cut on my bigger Chinese lathe.
Next I will bore the barrel to 63mm, then turn the exterior of the barrel.