Zinc pretty much ruins a good casting alloy, in my experience. It changes the surface tension of the alloy, basically doing the opposite of tin. It causes moulds to NOT fill out sharp edges. The bullets are often undersized. You can do it with round ball moulds, if you actually want them hard and undersized, or some blocky bullets with basically flat sides that you want undersized. Bullets with any detail lose that detail. It seems like the zinc wants to migrate to the surface of the bullets and form a hard shell with high surface tension.
That's been my experience. I have had a couple successful experiments removing zinc from an alloy using copper sulfate.
