Barrel twist would indicate conical or sabot w/heavy, tough, bullet like a Swift, Partition, or Barnes. Black powder, 777, or Pyrodex, with BP and 777 probably the best, although I would be sure to get fresh 777 and test it just before the hunt. You might also consider using musket caps if you can find RWS ones. That will require a nipple swap, of course.
Spend what you have to for the best chance of success. This is no time to go cheap!
Best of luck.
