I discovered how to estimate maximum pressure in 9mm pistols by measuring case expansion with a dial caliper. The assumption is factory 9mm ammo is loaded to maximum pressure, therefore average factory case expansion is used to establish the max. 9mm cases are tapered, as are 9mm chambers. The higher the pressure, the more the case expands. In my experience with over a dozen 9mm pistols, 9mm cases tend to expand to .391" in standard loadings, more with +P. Federal 9mm +P+ expands to .394"/.395" in my guns. Of course it is best to use virgin brass for this, as reloaded brass suffers from work hardening and may give a false reading. This also does not work for straight-walled cases as they are, well, straight and will not expand more with increasing pressure. Why? Because the chamber is straight as well. I've been doing this for over five years now and the results are consistent and repeatable. Pipsqueak loads expand to about .388", moderate loads to about .390", and full-power loads to .391" or more. I stop at .391" because I don't need the additional power. I have loaded the 9mm to Major power factor, but the case expansion was in excess of .396" and primer pockets loosened with just one firing. It works for competition, but I'm not going there again. So, next time you are wondering how hot your latest recipe is, compare your case expansion to factory ammo fired in your gun. No, it is not perfect, but it is alot more reliable than looking at the "flatness" of primers.