The following is based largely on data published by Vulcan Lead and by Alchemy Castngs.   Most of the data points are for various solders because that is the only type of lead alloy for which tensile strength information is readily available.   If I run across more data I will add it to the chart and revise the results but in the meantime this will have to do.
Extrapolating the results of the chart to common bullet alloys gives us the following:
|alloy||BHN||estimated tensile strength (psi)|
By comparison, mild steel is 36,000 - 45,000 psi.   So what does it mean?   It means that almost all lead bullets will obturate to some extent at peak pressure -- but bear in mind that there are degrees of obturation -- how much obturation, how soon, and for how long.   It means that even the hardest lead alloys are soft and easily deformed compared to other metals.   You already knew that, of course.