Hmmm. Hand rubbed between each coat, I hope.
Seriously, this has potential, and potential pitfalls.
I have a golf-ball caliber hand-gonne that is a two part design - a follow on to the GB mortar contest.
As you recall, we developed essentially the same plan independantly at the same time.
Mine is slightly different in that I use an offset in the location of the trunnion/trunnion-hole to lock the pieces together. I built several each with a different feature or barrel length.
Moving to a larger diameter (gb to bc) opens up possibilities - that of being able to use many of the rounds/drops that are so easy to accumulate with a nominal purchase of DOM tubing. Perhaps a little more expensive in per-foot cost, but much less expensive in TIME to machine.
Hmmmmm. ...