Bullet seating comparator

Different breech pressure measurements


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Q:I have Hodgdon’s 2012 Reloading manual and there’s something about itthat puzzles me. Some of the chamber pressures are listed in CUPwhile others are PSI. What I want to know is: its it possible for meto estimate one from the other?

KeithMorrison

 

A:Normally, if two variables are correlated, you can estimate thedegree of relationship from one to the other. In the case of CUP andPSI. The snag with the two different measuring systems is that theyboth contain an error in the form of “standard deviation”(SD). SD of the CUP measuring system is 2000 PSI (correct usage) andthe piezoelectric system is slightlymore accurate with an SD of1300PSI. For example, the 7×57 is 46,000 CUP and 51,000 PSI. SD iswhat causes the apparent inconsistency between the two measurements.But there are variables in both systems that make precise readingsjust about impossible. Denton Bramwell in his book “CorrelatingPSI and CUP” lists two formulae for converting CUP and PSIvalues back and forth. The first formula is for ANSI PSI and CUPvalues as used by SAAMI in the U.S: ANSI PSI = 1792.0 + 1.151586 xANSI CUP. But CIP in Europe uses a different system and a differentformula for accurate conversions: CIPPSI = 2806.88 + 1.20911 xCIPCUP. Check them out on your calculator.

 


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