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SAAMI says it another way Brent:
"Maximum average pressure for 2 3/4" and 3” shotgun shells will be 11,500 PSI +/- 900 psi, with some maximum extreme variability not to exceed an uber-max of 12,500 psi +/-900 psi"

So a SAAMI standard shell may actually run 13,400 psi and that would be the "Maximum Individual Service Working Limit" or per CIP "Maximal Statistical Individual Pressure"

And the pressure we should aim for is the "Standard Service" pressure.

Vic Venters, Shooting Sportsman, March-April 2012, “CIP Proof”
“Although the Maximum Mean Pressures for service loads for standard proof (850 BAR) guns are 740 BAR (10,733 psi), CIP regulated cartridge manufacturers typically work to lower pressures...between 450 BAR (6,527 psi) and 650 BAR (9,427 psi) as measured by CIP piezo transducers.”

Pressures published by Burrard in 1955