I found an article on SOFREP that stated that the 7.62 NATO measurement is in CUP and that the difference in pressure between 7.62 and .308 is 58000 PSI vs. 63000 PSI, not a significant difference.
"This is where things get a touch confusing. The maximum pressure numbers for these two calibers are typically shown to be around 63,000 PSI for the .308 Win and just 50,000 PSI for the 7.62. That may seem like a significant difference and grounds for people to think it an ill-advised move to fire a higher pressure .308 cartridge in a rifle that’s fitted for the 7.62 NATO. But hold your horses there, fella. It really isn’t that simple.
The truth is that that so-called 50,000 PSI is not accurate, at least not in terms of pounds per square inch. 50,000 is more accurate if you’re talkin’ copper units of pressure or CUP which is a whole other animal. CUP is an inferior method of measuring pressure which depends on looking at how tiny an amount of copper disks compress when you fire your weapon. The approximate difference between CUP and PSI works out to about 8,000 which I think we can agree is a giant leap in terms of rendering a precise figure.
To put it another way, the max pressure for the 7.62 NATO is somewhere closer to 58,000, placing it closer to the PSI of the .308 Win than one might be led to believe. The 4,000 difference is hardly a difference at all, at least not one that would disrupt the trajectory of a shot aimed at a sluggish boar."