Mike,

If you were dealing with British loadings you would have 3 different loads (for each case length):-

1. .577 Black Powder Express (lead bullet, probably paper patched, comparatively slow rifling twist)
2.,.577 Nitro for Black Express ( for use in 1. above - lead gas checked bullet )
3. .577 Nitro Express (jacketed bullet, much higher pressures, will FIT in 1 &2 above but unwise probably dangerous to use)

Rifle made for 3 above will be stronger, heavier, probably faster twist and noticeably thicker across the breech ends)

What does yours weigh? What is the twist?