[WayneShaw]

What advantage does a small percent of antimony give in a core? I've read 1/2% to 1% is used, but what is the reason?

Searching here, one reason stated was to reduce the cored jacket from sticking on the punch. This makes sense. There is a fine line, no pun intended, between a punch too big that scrapes the jacket, and a punch that has clearance. Clearance means some lead squeezing by the punch. Pure lead flows easy, and it doesn't take long to get a bit of lead buildup on the punch then the core jacket sticks on the punch. With a little harder alloy, maybe you can get the pressure to fill the core and not have the issue of bleed by.