A proof pressure of 1100 kg/cm2 = 15,646 psi, for a service load pressure of about 10,000 psi
The pressure numbers were however obtained using lead crushers, and pressure derived by modern piezo transducers would be 10-14% higher.

I would be very cautious about lengthening the chambers of a light weight continental 20g, and then only by an expert like Dean Harris at Skeets Gun Shop
Tahlequah, OK
918-456-4749

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