There are several things of interest in this thread. One thing I would like to make very Clear though is there is a lot more involved in getting the shot load from a standing position in the shell, out of the muzzle with useful velocity than just the max or peak pressure. In Fact Max pressure has Absolutely "NOTHING" at all to do with that thin spot 22" down the bbl (30"-8").
Look at those 4 pressure curves recorded by DuPont back in the early 1920s. Note that those curves all swap places at around 2½"-2 3/4" from the breech. Beyond there the lowest pressure curve becomes the highest one etc. Though these curves are only carried to 10" it is seen that by this point they are all close together. The biggest gap her is the "Higher" pressure at this point of the shell bearing an "Underload" of a Slow Burning Progressive Powder". Those of who tout your very low pressures using an underloaded slow powder might well bear to keep this in mind.
Another point of extreme interest is the vast difference in pressires recorded by DuPont at that time between those in the 2001 Lyman data reported. I do not currently have an explanation for those differences. It is noted that DuPont used 3F powder while Lyman reports with GOEX powder used 2F. For the nearest load comparsion though Lyman used 90 grains (3¼+ drams) while Dupont used 82 grains (3 drams). DuPont did not stated their velocity but a 3-1¼ load shouls fall in the 1150-1175 fps range. Lyman reported 1100 fps vel. Max pressure listed by DuPont was 9,000 psi, that by Lyman was 5,100? (perhaps LUP). Could the difference in 3f vs 2f, PE vs LUP & perhaps a higher quality powder in the 20s result in a 3,900 difference in max pressure, I am just not sure.
While true, the bbl really doesn't know or care if the pressure upon it comes from BP or Smokeless there is one major difference between the two. That is their abilities to burn. Black powder will burn about as well unconfined as under pressure. You can thus load BP down to as light as you like withoput fear of a squib. One needs to keep in mind that the slow progressive powders were designed "NOT" for low pressures, but for pushing heavier loads of shot or acheiving higher velocities (Or Both). They acheive this ability by being made Harder to ignite, which when loaded to pressures below their design rate can (& often Does) result in inefficient burniong, even to the point of an ineffictive load or a wad left in the bore, something to keep in mind particularly if you try to use them in colder weather.
