Jon;
Pressure curves have been posted here from time to time showing loads of the same ballistics, IE shot weight & velocity with various burn rate powders including Black. Peak pressures for all of them occur within the first inch of travel. The faster powders have a higher peak. The curves cross between from around 2 3/4" to 4" & the faster then have a "SLIGHTLY" lower pressure from there forward.
Note that total barrel time from ignition to exit will be on the order of 0.003 of a second. An acceleration curve begins with a steep climb upon ignition & gradually lessens as it goes down the barrel. All this would tend to lead to the conclusion that the total time the peak pressure could be higher with the fast powder at the Utmost would be on the order of 0.0001 of a second & for a total travel distance of no more than about a Ľ inch. It doesn't take a big difference in the pressure level for the rest of the barrel to offset this difference in peak pressure. Not enough to be concerned about.
Bottom line is that pressure is really only of concern in the chamber itself. Reaction, IE shot weight & velocity is what affects the stock & primarily frame stress. It was not the increased pressures that cracked lightweight Flues frames it was the increased back thrust from the higher velocity loads with heavier shot loads.
Using the "Super-Low" pressure loads will of course not harm the gun as long as it goes off adequately & clears the bore of all components, but also is not doing it a great favor. The risk of inadequate ignition does become greater though.
Using a sensible load of shot at a reasonable velocity level with a proper load of the powder which was "Designed & Developed" for this type of load is the best way to go, in my opinion. If the chamber area itself is not constructed adequate enough for an 8K load, shoot Black or Hang it on the wall or sell it to Cracker Barrel. I have followed this rule for some 40+ years now & have never encountered a problem thereby.
