More searching found this explanation as to why C.I.P/NATO 62,366 PSI equals SAAMI 55,000 PSI.
Which is different than saying 55,000 PSI equals 62,366 PSI. Kind of like comparing PSI and CUP.
https://en.wikipedia.org/wiki/5.56%C3%9745mm_NATO
Pressure[edit]
C.I.P. defines the maximum service and proof test pressures of the .223 Remington cartridge equal to the 5.56mm NATO, at 430 MPa (62,366 psi). This differs from the SAAMI maximum pressure specification for .223 Remington of 380 MPa (55,114 psi), due to CIP test protocols measuring pressure using a drilled case, rather than an intact case with a conformal piston, along with other differences.[52] NATO uses NATO EPVAT pressure test protocols for their small arms ammunition specifications.
Because of these differences in methodology, the CIP pressure of 430 MPa (62,366 psi) is the same as a SAAMI pressure of 380 MPa (55,114 psi), which is reflected in US Military specifications for 5.56mm NATO, which call for a mean maximum pressure of 55,000 PSI (when measured using a protocol similar to SAAMI).[53]
These pressures are generated and measured using a chamber cut to 5.56 NATO specifications, including the longer leade. Firing 5.56mm NATO from a chamber with a shorter .223 Remington leade can generate pressures in excess of SAAMI maximums.
