I have just noticed the advice from snuffy to add tin. Don't. Cast bullets don't need tin. It does very little for hardness, tin is for castability ie free flow properties, but alloy with 4 or 5% antimony casts well without tin. Antimony is the hardening agent for lead alloy. Wheelweights traditionally had 4%.
Wrong! Tin is necessary to alloy antimony with lead. Just antimony and lead without tin leaves crystals of antimony surrounded by soft lead. The boolit feels hard, but will lead horribly. Tin also toughens boolits, by the act of allowing the antimony to become a true alloy, instead of crystals of antimony separated by pure lead. If you want 6% antimony boolits, you MUST have 2% tin to allow the antimony to mix with the lead.
Wheelweights are always short on tin content. They average ½% tin. Adding 1.5 % tin will assure you have enough for strength and good sharp fill-out.
Dikko, welcome to the casting forum.