Most casters shy away from zinc in their mix but commercial ammo not so
Could you provide a source for this please? I'm pretty certain that is not correct, on the basis that I've cast with remelt from a lot of commercial bullets and never found any trace of zinc. Zinc messes up lead alloys so badly that I'm pretty certain no commercial manufacturer would use it either; even lead wire for bullet cores starts as a melt at some point, and zinc turns that melt to clumpy oatmeal.
I'm guessing you are thinking of antimony rather than zinc. Antimony (Sb) is a common element in lead alloy and is used to control lead hardness, both for casting and for swaged lead commercial bullet cores.
I got a tour of the Nosler facility a number of years ago, and being interested in lead and lead alloys, noticed they had rolls of lead wire (for bullet cores) labeled with different percentages of antimony; IIRC the highest was 12% which is quite a lot and makes a pretty hard alloy. There were also 2%, 8%, etc and I assume pure (0%) as well.