You may not be able to tell if Zinc is present while it is in the ingot form. However, a contaminated (Zinc, Calcium from maintenance free batteries, etc.), will cast a bullet that will fill the mold but as it cools, it will shrink away from the side of the mold and will drop from the mold distorted. The proof is in the casting.
Sometimes you get what you pay for, sometimes you only pay more for what you get.
Three shots are not a "group"...they are a "few".
If the Bible is the literal, infallible, unerring word of God...where are all those witches I am supposed to kill? (Exodus 22:18)