A bit of research turned up the following:
Shot containing up to 0.5% antimony is generally called “soft shot”. Shot containing
more than 0.5% antimony is known as “hard shot”.
This "hardness" standard is the same for buckshot pellets.
Regarding Remington lead factory loads:
STS/Nitro27 target loads 6% antimony
American Clay & Field target loads 4% antimony
Gun Club and Field loads 2% antimony
In the Remington LE Buckshot line this was the story:
"...our 9-pellet Express® LE Buckshot loading created a new standard of pattern performance with the addition of our one-piece compression section Power Piston® wad column, polymer buffering and hardened 3% antimony pellets
. The combination of these advanced features produce patterns 25% tighter than standard buckshot"http://www.remingtonle.com/ammo/ss_expbk.htm
Promoting a 3% antimony content for Remington LE buckshot then implies the standard consumer grade Express buckshot contains pellets of a lesser antimony content.
As for the "...extra hard, high antimony..." content of buckshot pellets in the Hog Hammer
line, Remington has not yet replied to my query.