Comparison of modern dense powders for 12g 1 1/8 oz. at 1200 fps (3 Dram. Eq.) in a Remington case with Rem 209 Primer and WAA wad from Alliant Powder http://www.alliantpowder.com/

Alliant e3 - 17.2 grains = 10,950 psi
Red Dot - 18.3 gr = 10,300 psi
Green Dot - 21 gr = 8900 psi
(Infallible) Unique - 22 grains = 8900 psi

Published independent testing:
3 Dr. Eq. 1 1/8 oz. Winchester Trap Load
1,202 fps @ 9,600 psi
AA Xtra-Lite 1 oz. WAAL12
1189 fps @ 8000 psi
AA 'Low noise Low recoil' 15/16 oz.
980 fps @ 6,200 psi

You have of course measured the wall thickness of the 3 vintage guns? And checked the head of the stocks for cracks?

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