Pure lead has a BHN of 5. Clip on weights are 99.5% pure and have a BHN of 6. Clip on weights are 96.5% pure and have a BHN of 10-12.
I posted this in the reloading forum awhile back but it didn't get much response. It's got a lot of useful casting information in it and links to even more info.
http://www.lasc.us/CastBulletAlloy.htm