Copper Unit of Pressure (CUP) Defined by Hodgdon

When you look at a loading manual with load data, you will usually see pressure ratings for stated load. Sometimes these are listed in PSI numbers, which most people correctly understand to be Pounds per Square Inch of pressure. However, powder-makers also commonly list pressure in CUP numbers. CUP stands for Copper Unit of Pressure. You may be asking — “What exactly is a CUP, and what is the origin of that unit of measurement?” You may also be wondering — “What’s the difference between CUP pressures and PSI pressures?” On Hodgdon’s Facebook Page, you’ll find answers to these questions.

Q: What is CUP?

A: Copper Unit of Pressure (CUP) is a measurement used in the ammunition industry to determine the chamber pressure created by a cartridge load. Originally, a precisely formed copper slug was placed in a fixture over the chamber. When the cartridge was fired, the amount of crushing measured on the slug allowed engineers to determine the pressure.

These days, modern electronic transducers provide faster, more accurate measurements of chamber pressures in pounds per square inch (PSI). CUP and PSI are measured to different scales and are NOT interchangeable.

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