Don't know about zinc vs mild steel, I'll wait for Sagacious to reply.
I am saving my zinc WWs for future use.
I ladle-cast with SS or enameled pots so it probably won't concern me, unless I hit the Lotto and buy an electric melter.
Still interesting though.
I figure sooner or later lead will be as popular and easy to get as, say, plutonium.
Between that time and the time that zinc is regulated out of existence I will learn to cast zinc boolits.