I would be rocking the M1899 like crazy if they put it in RDO. It's technically in the game, but it's stranded with a bunch of other cut content for a Halloween update we never got.
The M1899 is obviously the best, but I think the double action revolver should be an option since it’s probably the best REVOLVER you can get. Although the sawed off shotgun isn’t that great, it can be fun to duel wield them while in first person