In a 1927 Western Cartridge Co. flyer "Super-X The Long Range Load" by Capt. Chas. Askins, the 12g. duck load is described as 38 1/2 grains or 3 1/2 dram with a muzzle velocity of 1400 fps and a breech pressure of 3 3/4 tons or just below the current SAAMI 12g maximum of 11,500 psi.
BUT by 1900 1 1/8 oz. 3 1/4 Dr. Eq. Dense Smokeless shells were widely available, and generated (and estimated from period publications to transducer numbers) 9500 - 10,500 psi. 1 1/4 oz. 3 1/2 Dr. Eq. loads used Bulk Smokeless with about the same pressures.
