larry;
I think you are totally confusing "High Pressure" with "High Back Thrust". What these guns you mentioned were Beefing up for was the higher ballistics of a heavy shot load at a higher velocity, not necessarily meaning a higher pressure.
3 drams of bulk smokeless or 28 grains of Infallible or Ballistite pushing 1 oz of shot were not low pressure loads.
True these weren't the "Normal" but they may have been used far more often than we tend to imagine.
Max peak pressure is simply not the whole story in internal shotgun ballistics. 1 oz of shot @1 200 FPS with 10K PSI simply does not stress the actin as much as 1 oz @1300 fps with 9K psi, even though the pressure is lower for the heavier load.
Drew;
While in agreement with your figures loads of those days were found recommended with a 3 DE of dense powders with 1 oz of shot. Since the same powders were used obviously the pressure would have been higher than that cited for the 3DE-1 1/8 oz load. The 1913 Lefever Arms Co catalog shows those loads of 28 grains Infallible or Ballistite with 1 oz shot, same as Rocketman quoted, as recommended loads (Steel, Twist or Damascus, no distinction being made). The 3 DE of these powders was 26 grains. my understanding is the Infallible & unique are essentially identical powders, I don't know why it was marketed under both names.
