I have a similar stye of rifling in my Lancaster style flintlock long rifle, 44 inch swamped collerain barrel, the rounded grooves work fine w. your rifle would be for patched ball. Ball was for a long time a preferred projectile, hardened with a little tin or antimony. Big charge, and slow twist to prevent stripping. Lovely piece, I do not to envy, but I could !! You might try a greased felt wad over the charge and see how that goes. I have had success with my 12 bore Forsythe type rifle. It will interesting to see what charge your settle on, I think the FFg is a good idea. These rifles I suspect are for shorter, rather than longer ranges. For dropping large moggys at your feet !

Lovely piece look forward to your next range report.

Cheers

Gordon