One of the big problems with roundballs today is that many rifles have fast twist rifling designed to stabilize the big, heavy conicals.
Round balls stabilize best with a slow twist. IIRC the twist for .45 to .50 caliber is about 1 turn in 66 inches, where as with a conical in those calibers is around 1 turn in 24 to 32 inches.
Thompson Center rifles used to have a compromise twist, generally 1 turn in 48 inches, that would adequately stabilize both round balls and conicals, but which generally woudln't give the best accuracy with either.
After Pennsylvania adopted the round ball only, flintlock only rules for what they called the primitive season (designated muzzleloader season) T/C came out with the Pennsylvania Hunter rifle, a flintlock with a barrel that had the twist opitmized for round ball.
At my old club in Pennsylvania years ago I saw one guy land 3 of 5 shots from his .45 Pennsylvania Long Rifle on a 12" gong at 500 yards, and judging by the dust puffs the other two didn't miss by much. He was shooting a patched round ball and IIRC about 55 grains of 3 Fg.