"Chilled" is an advertising word used since the late 1800's to denote harder than normal shot. It likely means drop-shot and that's about it. All shot was made that way - back in the mid to late 1800's.
In my understanding, harder shot today, as Mikey says, has varying amounts of antimony added.
3% is common & I think what Hornady has in their buckshot sizes, with the hardest shot (Bleimeister- Alberta) having about 5% antimony added. The Bleimeister shot is the hardest, tightest patterning shot I have ever tested. It is better than any of the copper or nickle plated, or hard shot from the States. If you can find a bag of it at a shoot - buy it. Incredible stuff and good for 10 to 15% higher pattern density.
Haven't bought any for 20 years, so I do not know if they are still making it.
The use of modern plastic wads in ML's has been going on since the 70's that I recall. I did it and then had to remove plastic from my bore that had melted form the BP flame-out. I found that putting a hard card under the wad, would help reduce or eliminate this melted plastic. Uncut wads (card under) would certainly help with tightening up the patterns in non-choked bores. They might produce too-tight patterns close-in, and work best further out, say 40 to 60yards. One needs to experiment with uncut, then differing lengths of splits, like Standing Bear has.
