Well, I looked up the alloy data on the South West Casting site
(2% tin, 6% antimony, 92% lead) and then used this -
http://castboolits.gunloads.com/show...oy-calculators
lead calculator to look up the alloy. It was described as "Hardball Alloy" with a Brinell hardness of 16.
Here's something else to read about hardness -
http://www.rserv.com/Alloy.html
If you really want to shoot lead at high velocity, gas checks help. I believe the bullet mold has to be designed to accept gas checks -- they can't be added to a standard lead bullet.