X-Caliber,
Sorry I can't be of much help to you on this, but I have a similar question that maybe you can answer.
Have you ever tried a Maxi ball or Lyman's Plains bullet in the 1:60" barrel? What were your results? I'd be interested to know, as I also have a Lyman Great Plains Rifle in 50 cal, 1:60". Please see my most recent post, LGPR: Can you shoot a Maxi? I'd like to know your experiences, as I am in the process of putting my weapon together.
Looks like we are after the same information.
Thanks,
Matthew