Funny, this question was just asked over on the SASS Wire earlier this week. Here's what I wrote:
I always have to look this up to get it straight.
A small amount to antimony, from 1/2% to 6%, is added to shot to harden it. Shot is still made by pouring the molten lead alloy through a seive. As the droplets fall through the air, surface tension forms them into a ball, just like raindrops are ball shaped as they fall, not 'teardrop' shaped. The shot solidifies as it falls, and ends up in a pool of water at the bottom of shot tower, to cool. After it cools, it is run through successively smaller screens to separate it out by size.
'Chilled shot' refers to a German invention of blowing cold air across the shot as it fell. It was felt that the cold air made the shot harder. Later they realized that they were just adding more antimony, which made the shot harder.
Today there are three varieties of shot. 'Dropped' shot, that has about 1/2-1% antimony, 'Chilled' which has 3-4% antimony, and 'Magnum' which has up to 6% antimony.
The more antimony, the harder the shot. The harder the shot, the less deformation the individual balls get both from rubbing against the barrel, and from compressing against each other as the charge makes its way down the barrel. The less deformed pellets there are, the more predicable the pattern.
There are some hunting sports that actually prefer the Dropped shot, with the smallest amount of antimony. At close range with small birds, the deformed shot tends to widen the pattern a small amount without damaging the pattern.
In my area, I've only ever seen Chilled and Magnum shot for sale. In our sport, the targets are so close, it does not matter one iota which type you choose.