When you start casting bullets, your alloy is an unknown quantity. As long as you are casting for black-powder, either muzzleloading or black powder cartridge, the hardness of the alloy isn't really an issue as long as it is soft. Pure lead is best, and if you don't have pure lead, then softer is better.
Most of this information is couched in subjective terms because a lot of what we do is subjective. We don't have the tools at our disposal to absolutely measure hardness, so we use broad generalities.
Lead hardness is subjective in itself. We use Brinell Hardness numbers or Saeco hardness numbers. Generally, pure lead runs about a 5.0 on the Brinell scale and wheelweights run from 9.0 to 15.0 (or so). Water-quenching the hot bullets right out of the mold will raise the hardness to 22.0 or thereabouts, and heat-treating the bullet will normally raise the hardness up to 28.0 or better. This all depends on the amount of antimony in your alloy. Most of us have no good way to test lead hardness. The LBT company used to make a good little tester, but they went out of business. Saeco makes a bullet tester, but the results are in Saeco units and most of us use Brinell hardness in normal correspondence.
So, a lot of our work is guess and test and we learn that generally we want hard bullets when we are driving them fast and soft bullets when we are driving them slow. That suits most of us, but there comes a time when we want to know what we are driving down our barrels. Thankfully, Lee Precision has come up with a way to test lead hardness and made it affordable.
PawPaw has ordered one for himself. Soon, I'll be able to test the hardness of alloy with some sort of accuracy. Then I can tailor the load for the bullet, or vice-versa. The guys over at the Cast Boolit forum say that this is a dandy little tester and I'm looking forward to wringing it out.
The very first bullet I am going to put on it is that (*&^ little Meister bullet (#RB-32-20) that is giving Junior and me such fits. I want to know how hard that little $%^&* is so we can get it to shoot.