a. The chemical composition does not form a unique chemical signiture, becaus on cooling and solidifying, impuriites freeze out very non-uniformly - a fact that any semi-competent metalurgist would know. "Bullets created from the same block of lead can appear unrelated and bullets produced years apart can appear virtually identical."
b. "FBI examiners have testified that bullets are often smelted from batches of lead as small as 70 pounds — enough to make about 10,000 .22-caliber bullets.But according to the study, ammunition is usually created from lots of lead that weigh 50 tons or more and yield up to 17 million .22-caliber bullets — a number that would dramatically dilute the significance of any match."