The subject gun

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Manufrance Ideal 16g with the same proof marks; at 1100 Kg/cm2 = 15,646 psi + 10-14%. Service pressure would be about 2/3 proof pressure so about 11,500 psi which is the SAAMI max.
As Ted suggested, using 2 3/4" loads in a 2 1/2" chamber raises the pressure an average of 700 psi.
The 12g load for Standard T was 2.2 grams = 34 grains = 3 Dr. Eq. with 1 1/8 oz. but as noted the gun is Superior T Proved
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=618062

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Acier Hercule was a Manufacture Francaise d´armes tradename and I don't know the source nor composition.

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