Larry & all;
I have never seen anything to indicate that any major changes would occur to the pressure curve beyond that 10"-12" point. A careful study of the DuPont chart shows the two places of greatest difference between the curves occur first within the chamber at the 1"-1.5" point, folllowed by a point around 4" down where the curves are well reversed, IE the Low pressure load is now the high pressure one & vice-versa. My thinking here is the curves will maintain there relative positions from the 10" point to the muzzzle with a continuing decline to a pressure somewhat below half of what they show there. I would not expect another reversal. It is also noted the curves are very close together by this point. That thin spot @ 22" down the bbl is not really going to know, or Care, which load was fired through it, though what slight difference there may be will at this point favor the faster powder which started with the highest pressure.
In accessing loads it "MUST" always be borne in mind it takes a given amount of work to get a given load of shot to a given velocity. If you push a given amount of shot out the bbl at the same velocity then the total pressure area beneath the curve is going to be essnetially the same.
The only way to reduce the total impact on the gun is to load a lighter shot charge, A slower velocity or both. For most of my use I need no more tan an ounce of shot & never load it in excess of 1200 fps, usually a bit less. I then prefer to use a powder which gives a peak pressure within the 7K-8K range. This puts that pressure peak Right under the thick chamber walls where the gun was designed to take it. I then don't have to worry much if I'm shooting on a hot sweltering day or shivering from the cold I can expect reasonable performance under either condition.
