First, ditch the Maxi-balls. Can't imagine it will shoot them accurately with the slow twist your rifle has. Then get a Minie-ball sized .001-.002 under bore size. I get mine from Pat Kaboskey (Civil War Bullet Man), and he will size them to your specs. Using a generic .575 Minie in my rifle, bore size .581, I get "groups" of about 2 ft. at 25 yards. Using a .580 Minie, i get cloverleafs at 50 yards, and around 2" groups at 100. As far as charging goes, 60 grains of whatever reasonably appropriate powder will usually work fine. Mostly nowadays I use Swiss 3f in my original Springfield and Colt Special Musket. I get around 1070 fps with that load. Goex is much weaker, but works to just sling lead, giving about 980 fps. That would actually approximate the original loads from the 19th century. I also use Pyro P with good results, but Pyro RS gives dismal results in my guns, as it seems not to expand the Minie to engage the rifling. PRB's may be an option, as my rifles shoot them very accurately, but they hit 24" above POA at 50 yards.