I suspect you're confused by the change in pressure measurements that occurred, relatively slowly, over the past 50 years or so.
For a long time copper-crusher test barrels were used for measuring centerfire rifle pressures. These had a hole in the side of the chamber fitted with a steel cylinder, and when the round went off the steel cylinder crushed a copper cylinder slightly. Measuring the amount of crush provided the pressure reading.
It was assumed that copper-crusher pressure was pounds per square inch (PSI), but after the development of more accurate electronic pressure-testing systems, it was discovered the copper-crusher numbers did not translate directly to PSI, even though the pressures are listed as PSI in older manuals.
As the industry made the transition to electronic pressure measurement, copper-crusher pressure numbers were changed to CUP (copper units of pressure). Some new sources of data still list both CUP data alongside PSI (electronic) data, but eventually all the CUP data will disappear.
While there's no simple rule aboutthe relationship between CUP and PSI numbers (though a formula can make the conversion), it turned out that 50,000 "pounds" with the copper-crusher testing amounted to just about 60,000 PSI in electronic testing. Which is why the old industry-standard maximum for the .30-06, for instance, was 50,000 and the new industry standard is 60,000.
