Can anyone explain why some loads are measured in CUP and some in psi? Is there a relation between the two measurements? CUP is a copper slug?

ETA: "How I measure my LOAD."
































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CUP is how things were done before electronic transducers.  As there is no direct correlation between CUP and PSI, SAAMI can't just say "great, now cartridge X with Y CUP should be Z PSI."  New cartridges are being done in PSI, and over time, some of the older ones are being re-done to use PSI instead of CUP.

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Copper unit of pressure. Here wiki it, CUP

copper units of pressure

CUP is Copper Units of Pressure.  It is an archaic form of pressure measurement that uses a copper cylinder which is crushed in a tester by the chamber pressure.  Length of the cylinder is measured and CUP extrapolated from that.



There is no accurate comparison between piezoelectric transducer and CUP.  Piezoelectric are calibrated via deadweight tester and ARE a pressure measurement.  Some cartridges are close in PSI-CUP relationship.  Others are not.



SAAMI used both CUP and cartridge case deformation in developing the pressure limits.  This is why the 6mm Remington can go to 65,000 PSI now where the .243 Winchester is limited to 60,000 PSI.