Zinc hurts the cast finished outer surface and
changes the release from the molds. It makes an inferior product that does not have the same size as alloys without the contaminant. You may be able to make buckshot with some in it, but it may cause you pain and suffering as you go. It changes the specific gravity and can cause holes in bullets due to surface tension changes, in the very least it makes for rougher outer surface of bullets. Zinc is more reactive and does anode action in uneven mix, so the finish product can corrode if enough is present. Of course all metals corrode if left long enough, zinc is more corrosive potential than iron.
Even though the melting point of zinc is different than lead, the two mix and cause a poor melt. This may be why the bullets made with zinc mix are non-uniform. It takes too much heat to melt the zinc properly and by then you have lead too hot to be around.
Some people say zinc alone can be used. It has different properties. For buckshot there may be a certain amount of zinc that you can tolerate, but it probably is not a lot especially if you cast some with pure lead and get use to the way it releases from your molds.