They used to use lead "crusher units" to measure pressure developed by different loads. Lead slug of a known density were put in the machine and the amount of "crushing" created by the load was given as the pressure indicator. I believe they went to psi because it was more accurate.
I think the pressures are still listed in LUP's because that is the way they are measured. A classic case of the technique used for measurment influencing the numbers you get. What they are reporting is the estimated PSI to produce the observed amount of crushing of the lead pellet, not the PSI directly measured with a gauge or something. So it is probably more correct to report LUP's.
Well, psi, lup, and cup are all different indexescalibrations for detirmining pressure, what ever method/type of calibration you prefer. Since we are in an English speaking country we typicaly measure pressure in psi, Pounds @ Square Inch! Typicaly it is hard to get pressure gauges that can accurately measure psi in a shotgun, handgun, or rifle barrel/chamber. Copper units are similar to lead units, just not the same. Lead is/was tyicaly used in shotguns since pressures normaly run somewhat lower in shotguns than in handguns and especially rifles. A pellet of a specific benchmark of hardness is used in a special test gun whether it is lead or copper. Lead is much softer than copper so it would be used in shotguns. A copper pellet wouldn't even begin to squish in a shotgun test barel, and a lead pellet would be blown completly flat, non usable for measuring if it were used in a rifle test chamber. After fireing the pellet is measured, and depending on what that dimension is that related to some specific unit of pressure, either lead or copper. Neither lead or copper has a direct relationship to PSI or to each other. 15000 lup is 15000 of lup! 45000 cup is just that 45000 cup, Not 3 times what 15000 lup is or anything of the sort. Now days there are neat little gizmos called pezio tranducers that can measure strain on a firearm chamber and they are calculated and calibrated in PSI, not LUP or CUP. All three calibrations are relative to pressure, but they are not in a direct relationship like you have with fairenheit and celcius temprature callibrations. Simple! Right?
I'm not going to go into detail as to how the pellet is actually crushed or how the transducer is attached, but sufice it to say it is done quite specificaly in each situation. Don't try to interpolate one into the other, it doesn't work that way, even so some situations seem like they just might. For every one that does, 3 more don't. Lup and CUP is somewhat similar to the Richter scale. A rule unto it's self! 20,000 cup is not just 1/2 of a pressure that measures 40,000 cup. Same with lup. Just a guessing gauge!
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