Thanks Pete;
A few things worthy of note I think.
1st; for those not familar with it Oval was an early DuPont "Progressive" powder developed for the higher velocity type loads with heavy shot charges. It was noted in the article in this incident it was used in an "Underload".
2nd; the max pressure for all loads is between 1-1½ inches from breech & are within a 1K psi range of one another.
3rd; The maximum pressure variation occurs at about the 4" point where there is about a 1½K psi varition, with the highs & lows being reversed having crossed @ around the 2½" point.
4th; By the 10" point all are well below 4K psi & falling.
A few conclusions, shotgun bbls are designed by their makers to account for the pressure peak falling within the chamber, both black powder & smokeless powder guns, though with the switch to smokeless many guns were increased in their "Breech" thicknesses, even during the damascus days.
Personally on my damascus guns which are deemed in a condition to shoot at all, I am quite comfortable with 12ga loads in the 7-8K psi range & for 16 & 20 ga in the 8-9K psi range, and consequently use a powder designed for the wt range of shot for the gauge being used.
To take a powder designed for 1¼oz or heavier (12ga) loads & load it to extreme low pressures with only an ounce of shot is asking for failures.
I am aware many will state their success with such loads, my experience differences, namely in a duck swamp with temps in the low 20's. Once cured me of the habit.