Perhaps the greatest area of misunderstanding
of pressure terminology, has arisen from earlier practice of referring to
chamber pressure as some number of "pounds per square inch (psi)". Many earlier
ballisticians believed that results of a firearm chamber pressure tests were
accurately described as psi. The truth was their test results had little
to do with actual pounds per square inch.
Their "crusher" testing and the testing still used in many ballistic labs, uses a barrel with a hole drilled into the chamber perpendicular to the bore's axis, usually 1" from the rear edge of the chamber. A slip fit piston is fitted into this hole with its end contoured to precisely fit the inside of the barrel chamber. In testing, a cartridge is loaded into the chamber and the piston slipped into place. A copper crusher is then stood on top of the piston and is securely held in position with an anvil. When fired, the cartridge case has a small disc rupture from it at the location of the piston hole. The hot and rapidly expanding gasses in the chamber push equally on the bullet base and on the piston base. The piston in turn moves heavily on the copper crusher, which is forced to collapse to a varying degrees depending on the total amount of pressure applied to it by the piston.
The amount of "crush" of the copper cylinder is then measured carefully and this crush length is compared to a tarage table which lists a specific value for the amount of crush which occurs.
There is, of course, more to the crusher pressure testing system then the foregoing few words might suggest. The point to be made is that the copper (lead for shotguns and some handguns) crusher method is a valid and useful tool for ammunition evaluation, but it does not actually express pressures in true pounds per square inch. This did not escape the attention of newer generation ballisticians. They set out to correct the misnomer of pounds per square inch and used instead the designation copper units of pressure (CUP's) or lead units of pressure (LUP's). However, the erroneous term PSI had become so accepted that it was frequently used interchangeably with CUP or LUP. While such use was technically wrong it created no major problems when everyone was talking about the same thing; the result of a copper or lead crusher pressure test.
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