Originally Posted by
bigbulls
The 40S&W is not what I would call a high pressure cartridge. It operates at the same MAP of 35,000 psi. that the 9mm and .357 magnum operate at.
You're defining "high pressure" differently. The 40S&W is typically loaded right up to the max; that is not why there is a real "+P" version, it's already maxed out. Apparently after doing some reading it appears the few "+P" loadings are ABOVE max.
Conversely, the other loadings you mention do not typically operate at max. A 9mm not labeled "+P" is not 35,000 PSI. So there is more room for error if for example a standard 9mm round has a bit of bullet set-back from being chambered multiple times.