I'm wondering if he cast them. First of all, the melting point of bronze is like 3 times that of lead. The heat above/around a backyard furnace rigged to melt bronze is pretty ferocious. I know from doing sand casting for custom boat parts. The second question that comes to mind is how would you cut the sprue on a mould filled with bronze? That stuff is harder than you think and I can't imagine a sprue plate, even a thicker custom one, holding up for more than a couple of bullets. I wonder if they weren't lathe turned like the other gentleman's friend did.