I think you should try some wads. They can make all the difference in the world in some rifles. I really like my Hornady Great Plains conicals. I shoot the 425gr in my 54cal Renegade with a 1-48" twist.

If I were you I would buy some over powder wads. They might make a big difference. Also try some other powders like Goex or Pyrodex. There are many things you can do before you buy a new barrel.