The chamber pressure of the .223 and the 5.56 are both 52,000 cup or 55,000 psi.
This is one of the subjects I have spent an inordinate amount of time researching. If you dig and dig and dig some more, you'll find electronic copies of military specifications and standards.
Based on what I've been able to find so far, NATO uses pressure measurement techniques specified in the EPVAT standards and the method is very similar to the CIP methods. These numbers tend to run around 62000 psi.
The US Military uses the SCATP standard. The pressure measurement technique specified therein is very similar to the SAAMI pressure standard. Up until fairly recently these numbers remained around 55000 psi. I say "until fairly recently" because I have found that the US military has upped the pressure limits somewhat. An example would be the M855A1 round, and I think they've allowed pressures of over 58000 psi as measured using SCATP methods.
Then again, when you get to looking at all these different documents there are ranges of allowable pressure variance, and it gets a bit complicated for a non-statistician such as myself.
Here is a list of the document titles I have found so far (google may be your friend), if anyone's interested:
MIL-C-9963F
MIL-C-9963F_AMENDMENT-1
MIL-C-9963F_AMENDMENT-2
MIL-C-9963F_AMENDMENT-3
MIL-C-9963F_AMENDMENT-4
MIL-C-9963F_AMENDMENT-5
MIL-C-9963F_NOTICE-1
MIL-C-63989A
MIL-C-63989A_AMENDMENT-1
MIL-C-63989A_NOTICE-1
MIL-C-63989B_AMENDMENT-1
MIL-C-63989C
MIL-C-63989C_AMENDMENT-4-1
MIL-C-63989C_NOTICE-1
MIL-C-70460A(AR)5Oct1984
MIL-C-70460A(AR)15Oct1999
I think these all relate to the subject of 5.56 ammo, but I did not open them all to verify. Not on this junky old computer! If some of the docs are not relevant to the subject, hopefully you'll find them to be interesting anyway.
If someone uncovers more such stuff of an authoritative nature, I'd be most interested in taking a look at it myself.
