A chart showing 4 pressure curves all run with an identical 1oz load to a 3 dram equivalent velocity has been posted here numerous times. This was run by DuPont back in the 1920's as I recall. Pressure curves are shown out to 10" from breech. Of note is the two points of maximum spread between the curves is first at the peak point inside the chamber, about 1" from breech. The curves all then cross & switch positions about at the end of the chamber. The 2nd point of max spread between them then occurs at from about 4-6" from breech. Now pressure is considerably lower at this point, "BUT" bbl walls are also considerably thinner here as well.
My personal take on older guns is to restrict the amount of shot used as well as its velocity to just what is actually needed. I then try to use the fastest powder that will keep pressure within a 7K-8K range & figure I am doing the gun the best favor by keeping as much of the pressure as possible within the chamber where its walls are thickest. Also I don't generally have to worry about Cold Weather Squibs this way.
