Copper Units of Pressure. The units by which cartridge breech pressures are measured through compression of a copper crusher gauge. Similar but not identical to the older pounds per square inch (psi) system of measurement. Copper units of pressure or CUP, and the related lead units of pressure or LUP, are terms applied to pressure measurements used in the field of internal ballistics for the estimation of chamber pressures in firearms.
C.U.P. vs PSI Copper Units of Pressure