Well, after much thought, I think I will go with the .44 Colt rounds for my reloads, instead of the .44 Russian. All the ammo I have is .44 Russian, but .44 Colt brass is no more expensive. I feel better about using .44 Colt in the Open Top (and the R-M) than using .44 Russian in the 1860 R-M (and Open Top). While .44 Colt is certainly correct for the R-M, on the other hand, .44 Russian is less correct for the Open Top (since only a few were made.
I think it would be more feasible for a Open Top cylinder to be reamed for a .44 Colt (cylinder size big enough), it would be impossible to re-chamber (historically speaking) a R-M for .44 Russian because the rims are too big for an R-M cylinder.
It is a silly thing, I know, but.....