CUP and PSI are not synonymous. CUP (copper crusher method) can be considered as being somewhat of an average peak chamber pressure estimate rather than producing an accurate instantaneous peak pressure measurement. It dates back well into the 19th Century. For quite a few years, the ammunition manufacturers have been using a piezoelectric gauge which does allow precise measurement of an instantaneous peak pressure in PSI. SAAMI still recognizes CUP, but in fact there is probably no ballistics laboratory on earth today that still measures chamber pressures by using copper crushers. It is an obsolete technology which is comparable to attempting to tell accurate time with a sundial. Even though there is really no reliable way to convert CUP to PSI, or vice versa, you can assume that the actual instantaneous peak chamber pressure in PSI will be around 20-25% greater than the corresponding CUP measurement for the same load. There is indeed a correlation between the two, i.e., a high CUP indicates a high PSI, but the difference between the measurements taken using the two methods side by side has considerable variability depending on test conditions.

Another advantage of the piezoelectric gauge is that is far faster than using copper crushers. You can probably make 100 piezo gauge measurements in the time it takes to make one copper crusher measurement. There are many time-wasting operations to be performed in using copper crushers, and nearly none using piezo gauges.