Interesting Casper's.577 shoots such a heavy bullet while having a round ball rate of twist. Annomolies do exist.
Even the best .58's, the Enfield .577 ML's shot best with a 48" twist. The 1853, 3 bander had a 78" twist. The 1858 and 1863 both had 58" rot's.
The 1/4 turn of rifling was Forsyth's idea for the nest rot for 16 through 14 bore rifles for round balls.
Not suggesting anything, just stating what has transpired before.