Hardcast usually shrink more when cooled and may be a bit loose
I missed that the first time around. Actually hard lead shrinks less than soft lead so balls made from hard lead will be oversize and harder to load.
I don't see the problem with a little zinc in cartridge bullets. After all Zinc is softer than copper.
If you don't want the zinc keep your melting pot under 787 degrees and skim the Zinc off with the clips.