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Copper units of pressure or CUP, and the related lead units of pressure or LUP, are units used in the field of internal ballistics for the estimation of chamber pressures in firearms. These units, unique to the methods of pressure estimation used, have since been rendered largely obsolete by new pressure transducer methods of chamber pressure measurement.

Estimation of pressure using CUP and LUP units is performed by a special crusher gun, which uses a piston to crush a carefully manufactured and calibrated copper or lead cylinder. The amount of deformation is compared to the amount of crushing produced by different pressures in pounds per square inch. Copper cylinders are used at high pressures, such as are generated by most handgun and rifle cartridges, and lead cylinders are used for low pressures such as are generated by shotgun shells. The crusher gun has a hole in the chamber that is linked to a piston, and upon firing, this piston is subjected to the pressure of firing. The piston then acts on the calibrated cylinder, crushing it slightly. The length of the crushed cylinder is measured and compared to a chart of lengths resulting from crushing cylinders with given amounts of force, and the corresponding force is the CUP or LUP pressure value.

While CUP and LUP numbers were intended to be comparable to the crushing power of a given pressure in psi, the numbers are not equivalent! Since a longer duration, lower pressure pulse can crush the cylinder as much as a shorter duration, higher pressure pulse, CUP and LUP pressures frequently register lower than actual peak pressures (as measured by a transducer) by up to 20%. For example, the SAAMI maximum pressure for the 7.62 x 51 mm is given as 52000 CUP, or 62000 psi (430 MPa); the .45-70, on the other extreme, is listed as 28000 in both CUP and psi (190 MPa). SAAMI standards for a given cartridge may be expressed in CUP units, LUP units, or in standard units of pressure (psi or MPa).

Until the invention of measurement transducers in the 1960s, crusher guns were the only reliable method for estimating chamber pressures. Because there was no way to measure the deviation between actual peak chamber pressures and the pressure that generated an equivalent amount of crushing in a high pressure press, crusher gun numbers were often expressed as units of pressure--a bad practice even then, as CUP and LUP numbers are not interchangeable, due to the differing crush properties of different metals. With the availability of inexpensive, reliable transducers since the 1960's for actually making chamber pressure measurements, the industry almost universally has begun to move away from crusher guns for estimating chamber pressures, towards favoring making actual measurements. Transducers are also faster to use in practice, as they do not require the careful measuring of the copper or lead cylinders after firing. Additionally, transducers are capable of recording instant-by-instant pressures through the entire firing cycle. Hence, in the long run, using a transducer is less expensive, as it does not require using expendable metal cylinders in a crusher gun, and also reduces the labor required to analyze test results.

One outcome from this transition to using measurement transducers is, for example, that a Speer reloading manual from 1987 lists all SAAMI pressures in CUP, while current references list nearly all pressures in PSI. Another outcome is that design margins are now better determined, which has the effect of increasing the long-term safety of firing multiple thousands of rounds in a gun. With estimates based on crusher guns, actual safety margins could never be accurately assessed, short of actually firing tens of thousands of rounds in a sample gun.

• Chamber Pressure Measurement by Terry Hart
• Chart of SAAMI pressure levels for common cartridges, in PSI or CUP.
• A study done that shows a statistically significant correlation between CUP and PSI for rifle cartridges.

• Speer Reloading Manual Number 11, Omark Industries, 1987 (no ISBN)