Just an update. The 20 bore by 40 mm is almost certainly correct. I had measured as best I could and got a measurement of 2" to the start of the rifling. I cut down some 2-1/2" Cheddite hulls to 2" and they worked great. I even fired three of them. However, when I tried today with 20 gauge brass shells, the gun wouldn't lock up. I had to cut the brass shells off to about 1.85 inches to get a completely smooth closing action. The 40 mm would be around 1.585" and would allow the ball to patially protrude from the case mouth with a reasonalble BP load. I am going to shorten 10 or so to this length, load them with .646" pure lead balls at about 1100 and try for accuracy. The recoil of the 330 gr balls I tried ws very mild.
Thanks for the help. I new someone here would be able to dig up the answer.
