I should note that when I wrote what Steve quoted, I was speaking of 7.62/.308, not 5.56 mm. I also note I made an error in what he quoted. I said the CIP was measuring further forward in the case, but it is NATO that does that. The CIP uses 25 mm forward of the breech, as described earlier.
The
most comprehensive explanation of the unit discrepancy was written by board member FALPhil. It seems the military labs were using CUP but the manual editors changed it to psi for reasons unknown. Possibly they didn't know what a CUP was and thought they were making an appropriate correction.
The CIP puts the .223 Remington peak pressure limit at 3700 Bar (53,664 CUP) by copper crusher method, and 4300 Bar (62,366 psi) by Piezo transducer method (see
page 68, here). Hartmut Broemel's data in QuickLOAD for the 5.56×45 NATO has the same number. For an interesting discussion of pressure measuring read
this thread at the 24 hr. Campfirefor all the discourse between Denton Bramwell, Ken Oehler, and Hartmut Broemel as they discuss the matter and on some points disagree. One thing you'll learn is that Broemel writes the CIP's software and has access to more comparative pressure data than anybody else, so I expect the number he gives for the 5.56 NATO round is accurate by their measurement method for 5.56 NATO reference ammo.
So, this all begs the question, why would SAAMI and U.S. Military pressure numbers for CUP and psi differ so little, where the CIP has numbers differing so much more? Well, SAAMI seems simply to have copied the military numbers. No mystery there. As to the CIP and NATO having higher numbers than the U.S. Military, I don't have a good answer for that, but can speculate on a possible cause. The original reference loads would all have been developed on a copper crusher. The reference loads used by the U.S. and those used by NATO to determine Piezo numbers could have been different lots and therefore had different absolute pressure values because the copper crusher is so variable.
Below are the two samples of pressure measurement test data provided by SAAMI in their rifle and pistol documents. The upper numbers are for nine labs measuring the same lot of .30 Carbine reference ammo in copper crushers. The lower numbers are for seven labs measuring the same lot of .357 Magnum reference ammo using Piezo transducers. What jumps out is just how much more variance copper crusher pressure numbers have—over twice that of the Piezo transducers as a percentage of the average reading. Note also the two percentages with no column label on the right edge of the image. These are the percent the average standard deviation in pressure is of the average pressure. These numbers are about the same, indicating the consistency of the two different lots of reference loads were likely about the same. That means reference load inconsistency isn't accounting for how much worse the copper crusher consistency is. Clearly, one reference lot could be loaded 20% higher in pressure than another if different copper crusher units and different technicians did the charge determination.
