If you are going to be casting lead balls, you want pure lead. OFTEN, lead for industrial purposes can contain tin, arsenic or antimony(Sb). 2% to 4% Sb will harden the lead over time. A conical bullet that is too hard will not obturate properly. Balls, even those tightly patched, may not obturate properly. I usually buy my lead from Buffalo Arms. Yes, it is more expensive than harvesting, but at least you know what it is.