I recently picked up a handful of boolits on the surface of the range impact area.
I tossed them in the pot like usual, but 2 came up on top and wouldn't melt in.
I turned the pro-melt up as high as it would go and they still didn't melt.
They were obviously cast from a familiar looking .357 Keith SW mold.
They were WAY harder than normal cast boolits, and noticeably lighter.
I'm guessing they were Zinc.
If there is a practical way to cast it, it might be a useful process to know in the coming years.