Did a little looking around today and I still can find nothing official on the M855A1 cartridge, but I did find MIL-PRF-71208-AR online. The subject of the document is M995 armor piercing 5.56 ammunition, and it states:
The average chamber pressure of cartridges, conditioned at 70 F ± 2ºF (21 C ± 1.1 C), shall be not greater than 59,700 psi (411.6 MPa). Neither the chamber pressure of an individual cartridge nor the average chamber pressure plus
three standard deviations of chamber pressure shall be greater than 64,700 psi (446.1 MPa).
The document also refers to "SCATP-5.56mm (Heavy Bullet) Small Caliber Ammunition Test Procedures for 5.56mm Heavy Bullet Cartridges." This tells me that the pressure testing is done using the same pressure test methods used by SAAMI.
So, now I have seen a reliable 59700 psi average pressure specification. I still have seen nothing approaching 62000 psi in a military document, but I feel a bit more comfortable with warmer loads for my ARs knowing this.
