The correlation between CUP and PSI is fairly constant ,given the std. deviation allowable variable . ( Without it FORGET IT ) !.
I learned that way back in college as Math is an exact science and ALL experiments have variables . Again with Hercules it was as close as possible given Std. variables within a chemical batch on any given day . Exact NEVER happened super close YES ,sometimes .
MY rule 50K CUP /62K PSI , I know the Math rule is 50K CUP 60KPSI but I actually witnessed it on several occasions slightly exceeding the stated CUP . Just thought everyone should see the formula .
Another tidbit of info borrowed so I didn't have to type with link . Not all measurements are SAAMI ,either C.I.P. also gums up the works .
https://gundigest.com/gear-ammo/ammunition/223-vs-5-56
Let’s construct our own cartridge, just so we can remain theoretical for the moment. The “.30 Zoomer Magnum” has a maximum average pressure (MAP, or the allowed peak) of 50,000 CUP. We use the newfangled transducer to measure the standard reference load (in this case, 42 grains of “XYZ” powder under a 183-grain soft-point) and come up with 57,000 PSI. The “new” MAP for the .30ZM is now 57,000 PSI, where before it had been 50,000 CUP. But the actual pressure has not changed, we are simply using a new yardstick to measure it with.
Then we run into problems. In checking loading data, we find that some of the data wasn’t as “clean” as we thought. An example: using “123” powder under the same 183-grain soft-point, we had found that we could get 100 fps more and still only see 50,000 CUP pressure. With the new transducer and seeing things in thousandth of a second slices, we see that, yes, the main pressure peak is only 57,000 PSI, the allowed max by the new yardstick, but we also see a second, higher, spike from the bullet hitting and stalling in the rifling. That spike comes in at 63,500 PSI, well over the maximum allowed. So, we have to throttle back the load data, and all of a sudden “123” powder loses its 100 fps advantage.
There was no way to formulate an equation for a “universal translator” of CUP to PSI. Give it up, forget the conspiracy theories your gun club buddy tells you, just accept the new info for what it is.
The NATO spec for 5.56 has a higher “ceiling,” but it’s also measured slightly differently, and, again, there is no handy-dandy conversion. The SAAMI method measures pressure at the middle of the case. NATO (the European measuring group is known as C.I.P.) measures at the case mouth. A CIP-spec 5.56X45, measured at the case mouth, shows a pressure of 62,000. Measured at the case middle, as SAAMI does, it shows 60,000 units of pressure.