For a Jones-patent under-lever gun, 'mono-blocking' would probably be the only viable option, and perhaps the best one in any case.

sakmyk:
A very good suggestion. I tried to get Verney Carron to send me a pair of fully rifled 12-bore tubes several years ago. They didn't have an agent in Australia at the time, and were very reluctant to commit to anything. They played dumb a bit, demanding engineering drawings (!), and after several e-mail exchanges I could see the writing on the wall and gave up. I hope John303 has better luck, which might inspire me to try again... Still haven't built my own Paradox yet!