Actually, modern ammunition has nothing to do with it. The heaviest 12-gauge loads our North American Ammunition companies produced during the years Tobin doubles were being factory built was 3 1/2 drams of bulk smokeless powder or 28-grains of dense smokeless powder such as Infallible or Ballistite pushing 1 1/4 ounces of shot.
When progressive burning smokeless powder, high velocity loads, Western Super-X, Peters High Velocity, etc. came out in the 1920s they actually moved that 1 1/4 ounce of shot out at higher velocity but lower pressures than the heaviest old bulk or dense smokeless powder loads.
