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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.

CUP, LUP, and PSI are not interchangeable.
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