What's the throat on that ROA ? I have a relieved 430421 2C that drops .448+ , ideal for pp .451-2 . It is also is a resistance slip fit in a Dragoon . 35-40 gr of FFg should be pretty safe in that as well as 45ish of the home screen BP . I remember the ROA needing .458 RB vs .451-4 in the various Rem/Colts copies . You might get away with a 45-200 or the like in it .
I've tried 2-3 times to get a RB mould cut out to a flat base . EG a .454 RB with a .449 base and a fat lube groove . Dead soft it should be a firm slip fit on the base in most copies and positive seal on the ball . It would be heavier but no take up significantly more chamber space . The difference in 141 gr and 170+- and meeting the conical requirement might be just the ticket . Of course it might tumble and not shoot fer beans too due to insufficient length .