There was a chart of this nature showing the pressure curves over the first 10" of a 12ga bore done By DuPont back in the 20's I believe. It was made for Black Powder & 4 of Dupont's then current smokeless shotgun propellents. A copy of this chart with only 3 of the smokeless powders appeared in the "Rifleman" back in the '50's. I am not set up for picture posting but did a scan of the Rifleman chart & e-mailed it to someone here, forget for sure who, & it has been posted to this forum on several occasions. Perhaps whoever has it will see this & do so once more. Beyond that 10" point as the expansion area is constantly increasing there would be a slow but steady decline of the bbl pressure.
