PSI = Pounds per Square Inch
CUP = Copper Units of Pressure
LUP = Lead Units of Pressure
CUP is derived by using the copper crusher technique. A copper "crusher" is fitted to the rear of the chamber of the test barrel before firing. After firing, the deformity of the copper crusher is measured and compared to a chart. A chamber pressure is determined by extrapolating data from the chart. This method gives quick "ballpark" figures for chamber pressures. Copper is used for higher pressures because it takes more energy to deform. Lead is used to gauge lower pressures because it is softer and deforms much easier. LUP is the unit of pressure cited in many of the older shotgun manuals.
A modern ballistics lab uses special pressure barrels to measure PSI directly and more accurately. Pressures that are listed in PSI are more accurate than those in CUP or LUP.
As we proceed into the era of smart machines operated by dumb people, the piezoelectric method of measuring PSI, whether or not it is a "direct" PSI reading, is probably more accurate than prior methods.
However, in days of yore, when we preferred to depend on smart people with dumb machines, a good technician may have been more accurate, in relative terms, with CUP measurements.
In any case, peizoelectric PSI readings are generally higher than CUP readings for the same load.
Just to confuse things, you will see older statements which use the term PSI when referring to a CUP reading. In those days the terms were often treated as synonymous because there was nothing else.
"Just to confuse things, you will see older statements which use the term PSI when referring to a CUP reading. In those days the terms were often treated as synonymous because there was nothing else."
I know one guy, in the business, who says you should not shoot .308 Winchester ammo with a maximum SAAMI pressure of 62,000 psi in a military surplus 7.62 NATO rated for only 50,000 psi. What he does not take into account is that the mil-spec was written when crusher gauge readings were reported in psi, before the CUP term was thought up. Crusher gauge maximum pressure for commercial .308 is 52,000 CUP. No real difference. Heck, a .30-06 can go to 68,000 piezo psi. but has seldom been loaded above 48,000 CUP/crusher psi.