My suggestion would be to bypass the round ball and pick up one of the Lee 12 gauge 1 ounce slug moulds. They are really easy to cast. You use soft lead and fit in a shot wad put in a folded crimp case.
I cast nice easly to load slugs using R-P STS cases, HS-6 powder, Green Duster wad and Win 209 primers. Shoots pretty well in my smoothbores, don't have any rifled shotgun barrels.
Considering the price of these moulds, the cost of ten of those copper solids, it would be a nice investment.