My bet is that he put tin (usually from high SN solder or even scrap pewter) in the lead. It makes it fill out the molds better so you don't have voids in the bullets. Wheel weights are just about the right mix of lead alloy, with just enough antimony to get it hard enough not to lead up the barrel. A good mixture is 92% lead, 2% antimony and 6% tin.

If he doesn't powder coat his cast bullets, he should at least check it out. It's low cost, easy to do, clean, safer and beats the crap out of that nasty Alox or greasing them.