I will state a few points here as I have come to understand them. The Piezoelectric transducers give a virtually instantaneous reading so record actual PSI. Lead or copper (used for higher pressures if rifles)) crushers were calibrated to a known pressure & their amount of crushing measured.
Problem was the time of "Peak Pressure was so short the crushers were not reduced in length to the full amount of the actual pressure applied, thus they always read light.
As to Burrard's formula he was quick to point out this was not linear, but was close enough within the confines of ordinary shotgun pressures. I will add that I do not think we can guarantee that a 3 ton British & a 6,720 US LUP is actually identical. I do not know for a fact that both countries used identical alloys for their crushers or that the calibration procedure was identical.
Burrard's formula thus in my opinion can only truly be considered basically correct in relation to British crusher pressures.
I have seen some rifle data where both PSI & CUP pressures were listed. Some of the "Hotter" magnum loads showed a considerable difference while some old timers as the .45-70 showed only a slight difference. I doubt seriously that the variations are as great in shotguns.
