Hansol--- Yea, it is sage advice to listen to Daryl. The hardened round ball should be a tad under bore size. I have .726 bores and ordered a .724 round ball mould from Tanner.

Indeed Hansol there are more of "us" out there than you may know. Round ball shooters in smoothie barrels. Looking for stopper guns/take down a beast gun/hunting dangerous game gun. Two barrels and two hammers are the way to it.

I also shoot traditional muzzleloading black powder guns with round ball exclusively, well except for the shot loads in my 4 bore blunderbuss... Anyway, round ball accuracy is very satisfying doable. My 54 cal flintlock with round ball amazes me everytime I take it out and punch ragged holes at 50 yards with iron sites as long as I do my part.

I would think the .725 round balls available from Ballistic Products or many sources would be a great way for you to go. They are not hardened but they are readily available and cheap and it should get you a heavy round ball at .725 and you may find .725 will dramatically increase your accuracy. And for applications where a hardened round ball is not really needed a .725 pure lead round ball at 1400-1700 fps is still a very potent message.