I would do a cast of the chamber and a short section of the bore. See what the lands and groove measure. I would start with a ball of groove diameter. Patched balls were not the norm for bore rifles but I did get an odd ball bore rifle to shoot well using them. Sometimes you just need to keep fiddling with them. I like black as a propellant as well and many folks have found that Olde Eynsford

http://www.goexpowder.com/olde-eynsford.html

to be an excellent substitute for the old Curtis and Harvey #6 powder. Good luck with it, looks like a fun gun.
Steve
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ACGG Professional metalsmith, firearms import services.