Lead shot is classified by its antimony alloy content as either chilled or magnum. Antimony is the metal used to harden lead. Antimony is a brittle flaky, crystalline metal with a density of 6.7 grams/cc. Lead, when alloyed with antimony, becomes a harder pellet. As more antimony is added, the overall density of the pellet is slightly reduced.
Chilled Lead Shot is softer and slightly denser than Magnum lead shot. It is better for short-range applications. Many shooters, like Don Zutz, prefer the increased density of chilled lead for hunting loads. Chilled lead shot usually contains about 2-3% antimony.
Magnum Lead Shot is harder and slightly less dense than Chilled lead shot. Harder shot is more prone to retain its spherical shape during the firing sequence. Some shooters prefer Magnum's hardness advantage because it will likely pattern more predictably than softer shot. Magnum lead shot usually contains about 4-6% antimony.