I would not use castor oil as a lube. When it burns, it leaves a black-gummy deposit. I'd not want that in my bore.
What I have used that works well, is Track's Mink Oil, or Neetsfoot Oil. Neither of those leave a deposit and with a
snug patch, shoot quite cleanly, allowing subsequent loading without having to wipe or swab out the bore. For me,
a hunting load in a .50, would be 100gr. to 110gr. of 2F, or 85/90gr. 3F IF that load shot as well as the 2F load.
My .TC .50 shot best with 80/85gr. 2F, a .495" ball and .022" denim patch.