I have some experience with big bores and 1:66" twist. My jaeger with a Getz .60 cal liked .590" or .595" pure lead ball. .022" patch with either oil or spit, and 86 gr. FFg GOEX for target, and 120 gr. for hunting. I built the rifle with a folding leaf sight, but removed the leaf, preferring to hold front sight above the rear for long distance. I won several matches out to 275 yds. with this rifle.
My .62 cal Rice bbl. on my Hawken insists upon 127 gr. before I get the accuracy that will satisfy me, at 200 yds. I has again, a simple shallow "V" sight and bead front sight. I shoot a .613" ball and .022" patch in this rifle when accuracy matters. For trail shoots, I shoot a .600 ball...targets are a lot closer..