At the risk of repeating myself...I shall smile

In every U.S. maker vintage double that I have measured, a few with lengthened cones (of 1 1/2" or less), the wall thickness at the forcing cones was equal or greater to the wall thickness at the end of the chamber. The angle of the forcing cone is greater than that of the exterior taper of the barrel.

Apparent even in his low resolution image: Commission Internationale Permanente pour l'Epreuve des Armes ŕ Feu Portatives (C.I.P) Shotgun Minimum Wall Thickness Recommendation



Please note "Standard Steel" is defined as tensile strength 700 - 849 N/sq. mm = 101,526-123,137 psi (modern 4140)
http://www.cip-bobp.org/sites/default/files/new_file/A-4-1_EN.pdf
10 & 12g end of chamber recommendation - .079"
20g end of chamber - .075"

Pressure-distance curve shows pressure has fallen significantly by 3" for BP, Bulk and Dense Smokeless, and DuPont Oval


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