Since you wish to go traditional.. I would suggest a .54 caliber rifle because of the better ballistics of the roundball over a .50 caliber. If conicals and sabots are what you want to shoot a .50 caliber will work just fine.

As for the rifle, that's easy. IMO A Thompson Center Hawkins of Renegade. Go to the Auction sites and you can pick them up reasonable. I personally like the Renegades because they are a no frills type rifle, but the Hawkins are a little more understanding when shooting heavy conicals with recoil.

If the Thompson Center does not interest you then look into the Lyman Great Plains Rifle or the Lyman Trade Rifles. Both of them are well made and good shooters and will serve you well. They just do not have the customer service like Thompson Center that has spoiled most people...