Here is a chart I made showing the relationship between BHN and tensile strength for various lead alloys. Based on real measurements, not theory. Most of the data points came from Vulcan Lead and Alchemy Castings, just because they were nice enough to publish their data.

In other words, even the hardest lead alloy at about 30 BHN will yield at about 11,000 psi.

There are various formulas floating around on the web that would have you believe that lead is stronger than steel. A common one is PSI = 1422 x BHN. If you believe this formula, then a 30 BHN lead bullet does not yield until 42,660 psi.

**NOT**! ! !

Grade A36 mild steel has a BHN of 149. It's yield strength is 36,000 psi and a tensile strength of 58,000. Yet if we apply the PSI = 1422 x BHN formula, we're supposed to believe that the yield strength of mild steel is 212,000 psi ! ! !

On the other hand, let's apply my formula for tensile (not yield) strength, PSI = 375 * BHN + 500. My formula says mild steel should have a tensile strength (not yield) of 56,375 psi, pretty close to the official spec of 58,000. I'm not advocating using my formula for steel, I'm merely pointing out that it's in the ball park, unlike the 1422 formula.

To clarify, yield strength is when the metal begins to deform. Tensile strength is when the metal actually breaks apart. There's a big difference between the yield strength and the tensile strength of steel, but not so much difference for lead.

The pressure that burning powder applies to the bullet will not be constant, but will vary from the base of the bullet to the tip, like this:

What this means is that there are degrees of obturation. The bottom of the bullet may obturate, but the rest of the bullet may not obturate. Or the bullet may obturate when the chamber pressure peaks, but not before or after. That's life, we have to deal with it, and that' why I prefer to size bullets "big enough" so that they fit without obturating.