Probably less than the loads for which a 12g turn-of-the-century double was designed, and certainly less than the SAAMI max.
https://docs.google.com/document/d/1...UOZEFU/preview
1 1/4 oz. 3 1/2 Dr. Eq. BULK Smokeless was about 11,750 psi
1 1/4 oz. 3 1/2 Dr. Eq. DENSE Smokeless was 12,600 psi
Pressure is beyond the modern SAAMI recommendation of 11,500 psi
NONE of the turn-of-the-century barrel steels were designed for steel shot Kurt