Jim Watson's correct. The use of "PSI" back when copper units of pressure (CUP) was the only reliable way of measuring chamber pressure was a big mistake. But back then, folks from ballistic engineers to ordinary consumers didn't know any better because "pressure" was the key word and pressure per square inch was common in the USA for all sorts of things.
Download the following, print pages 16 and 21 then ask them to explain why the CUP and PSI numbers for the .308 Win. are not the same. And if the max pressure's really 50,000 PSI, why are numbers so darned much higher in SAAMI's specs for PSI? The difference between the NATO and commercial version is insignificant.
Here's some other sites with some good info:
Google CIP and check out what shows up.
For those who insist that USA arsenals loading ammo used the PSI system for measuring chamber pressure before about 1970, ask them what company made the transducers to measure them. If they give you a name, ask them how that could be because none were used before then. And finally, ask them to call Lake City Army Ammunition Plant and ask them when they switched over from copper units of pressure to the electronic psi systems.