Found this 9mm Luger ammo vs 9mm

[Kyrie]

Originally Posted by mrerick

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Nato standard "STANAG 4090" is a 1982 standard issued separately by NATO for their service cartridge. This specification is for a +P variant of the standard 9mm x 19mm pistol round with the same dimensional specifications as the 9mm Parabellum round.
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Respectfully, and with no offense intended, Annex C to STANAG 4090, Edition 2, in the section entitled, in part, "... NATO 9mm AMMUNITION" specifies the mean chamber pressure, transducer, "shall not exceed 230 MPa".

A quick conversion of 230 MPa to PSI yields 33,358.7 PSI. SAAMI maximum average chamber pressure for the (non +P) 9 mm Luger is 35,000 PSI

In short, STANAG 4090 calls for NATO compliant 9x19 ammunition to be loaded to a lower maximum average chamber pressure that that permissible under SAAMI.