mehavey, some years ago a friend ran tests on a rifle chambered for the .308 Win. Its barrel had a SAAMI spec chamber and bore/groove dimensions were very close to SAAMI specs. He used an Oehler System 43 strain gauge pressure and screened velocity measuring system. His strain gauge epoxied across the barrel about mid point on the chamber showed different factory .308 ammo and 7.62 NATO ammo had average pressure all within a 4,000 psi spread around 61,000 psi or thereabouts. The NATO ammo was a bit higher than the commercial stuff probably 'cause it had fatter bullets; as I remember what he told me.
But there is a little bit of apples vs. oranges stuff when different measuring locatiions are used relative to the cartridge case in the chamber across different test barrels. Not enough to matter much.