From a page on the Missouri Bullet Company, information along the same lines as your post.
Most cast bullet shooters don't know a lot about the properties of the lead alloy they're shooting because they haven't been educated about it. If you want to learn a little bit about some important cast bullet facts, then please read on.
A common conception is that when it comes to lead bullets, harder lead equals less leading. This is a false perception! To explain this surprising statement, it is necessary to discuss the physics of getting the bullet out of the barrel and how lead residue comes to be deposted in the bore. When the powder charge ignites, pressure is generated. This pressure is measured in “copper units of pressure” (CUP) and expressed in thousand of pounds per square inch. The heavier the powder charge, the greater the CUP. Naturally, the purpose of generating pressure in the cartridge case is to force the bullet out of the case mouth and on down the barrel.
Lead is a soft metal. Its hardness is expressed on a standard scale, called the Brinell Hardness Number (BHN.) The BHN of the bullet interacts with the pressure generated by the burning powder. The mechanism of this involves the effect of the generation of thousands of pounds per square inch of pressure which causes the base of the bullet to expand, or “obturate”. Properly obturated, the base will have expanded beyond its original diameter which has the effect of “sealing the bore” against the explosive pressure of the gases burning behind it. Properly sealed, and working in conjunction with the lubricant in the lube groove, the bullet will thus not allow gases to escape forward from around the base of the bullets, which prevents it from shaving lead from the bullet body and forcing it into the bore grooves (otherwise known as “leading”.)
This failure to obturate (“seal the bore against onrushing gases”) causes leading which is a chore to clean and is a major obstacle to accuracy.
An optimally hard lead bullet is simply one which obturates at a given pressure sufficiently to seal the bore against the gases which would otherwise “cut through” the soft lead (called “gas-cutting”, forcing molten lead into your rifling. A bullet which is too hard won't obturate and seal the bore, because the gas pressure is insufficient to expand the base of the bullet. A bullet which is too soft at a given pressure will experience excessive base expansion and vaporization of the lead, causing leading.
There is a formula for optimal bullet hardness which is simple and it is worth knowing:
Optimum BHN = CUP / (1422 x .90)
The CUP of your reloads is published in the reloading manuals. Take a typical .45 ACP load, using a 200-grain LSWC bullet – 5.0 grains of Bullseye. This load develops 900 FPS and is in common use among IPSC and IDPA gunners. The reloading manual shows that the pressure generated by this load is 20,000 CUP. So, the formula for optimal bullet hardness is
20,000 / 1279.8 = 15.62
There it is! For this application – shooting a 200-grain LSWC at 900 FPS requires that you use a bullet with a BHN of 16 to 18 (round upwards a couple of BHN points for flexibility.)
You may be asking why shooters don't know much about this whole bullet hardness optimization business. The reason is basically that the large manufacturers, for ease of production, use a standard alloy for all of their cast bullet construction, an alloy which has a Brinell Hardness Number of approximately 24. Why do they do this? It's simple – one standard alloy simplifies logistics for the big manufacturers and, equally importantly, a bullet this hard ships well by standing up to getting dinged around during transportation. The fact that their bullets are too hard and cause leading and aren't very accurate because of improper obturation is something they'd really rather you weren't aware of.
