Due to the veloicties involved, you'll most likely get the best kilng power and penetration by using round balls.
: Forget the slugs - more recoil with no benefits over round balls.
: There were two separate loadings in 12 bore for brass shell rifles. 150gr. and 191gr. - a moderate, and a heavy load. The ctg. guns, of course didn't leak gas at the breech, therefore gave higher velocity than will a muzzleloading rifle. It is a substancial loss, even though it might not 'feel' so bad. In this bore, a 200fps loss is signifigant.
: These were the round ball loads of the pre-slug era.