A thought as to potential accuracy with a .45 Colt fired in a MH bore.
Adapter issues aside: I seem to recall that vintage Remington, and possibly Winchester, factory loads used hollow based very soft lead bullets which likely "slugged up" enough to grip the rifling in a .577/.450 MH barrel and give acceptable accuracy.
Certainly not the same with modern hard cast or jacketed .45 bullets commonly loaded these days.

- Mike