The Commission Internationale Permanente pour l'Epreuve des Armes Feu Portatives standards were adopted in 1969, but I don't know when Spain became a signatory.
The CIP transducer Maximal Statistical Individual Pressure is 850 BAR = 12,328 PSI for a Maximal Average (Service) Pressure of 740 BAR = 10,733 PSI, and Mean Proof Pressure of 930 BAR = 13,924 PSI.

In Bell's study, the pressure increase of one 2 3/4" 12g load in a 2 1/2" short cone chamber was 1200 psi - it would likely be higher in a 28g

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