Mike: Tom Armbrust tried to pressure test some vintage loads a few years ago. The results (probably more primer deterioration than powder) were so inconsistent as to be meaningless. He did find a trend toward
increased pressure, felt to be from hardening of the old fiber wads and paper cases.
In a 1927 Western Cartridge Co. flyer "Super-X The Long Range Load" by Capt. Chas. Askins, the 12 gauge duck load is described as 38 1/2 grains or 3 1/2 dram with a muzzle velocity of 1400 fps (modern numbers are measured 3 feet from the muzzle) and a breech pressure of
3 3/4 tons or just below the current SAAMI 12g maximum of 11,500 psi.
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