OK, now I've got it. But I'm a word guy, and you techie types are still spreading confusion with the use of "peak". Pressure only peaks at one point in the barrel. Whatever the pressure is 6" or 12" or 18" down the bore, it ain't a "peak". It's just the pressure at that point. Look at it in reverse: a car accelerating from a stop. If it reaches 25 mph in, say, 3 seconds, that is not the "peak" speed of the car, unless it stops accelerating at that point. That's just its speed, as measured at that point. Peak is not reached until the point at which the car ceases to accelerate. Same for pressure within a shotgun barrel, but in reverse. If you use "peak" for anything but the highest pressure measured, I can guarantee that you will seriously confuse the non-techie public. It will cause them to believe that pressure within a barrel works like this: ^^^^^, going up and then back down numerous times, rather than like this: \.
OK, Larry, point taken. In the future, I'll refer to chamber peak pressure (which will be the single peak) with the term "peak." Peak pressure at other locations will be location maximum pressure. Now, just to be clear about what happens, as the wad passes any given point of the barrel, the pressure rises rapidly to the "location maximum pressure," and then decays to atmospheric shortly after the wad exits the barrel. How does that set of words play for you, Larry? Will it convey the technology to non-techies?
This is one of the things I love about this site. We can discuss in depth to get both the technology and the words right. Thanks for your input, Larry.
Rocketman, you're still using an unnecessary adjective. You don't need "maximum" either, and it can cause confusion. Pressure at ANY location in the barrel, anywhere after the pressure has peaked, is simply the pressure at that location.
No, it is the pressure at that location at that time/displacement. The pressure at location X will be maximum for X immediately after wad passage and it will be lower for X a split second later or fraction of an inch of additional displacement. There is only one pressure at 6", 12", etc.
Nope, there is some maximum and any pressure lower than that maxium and higher than atmospheric depending on time after ignition or displacement of the wad. "Maximum", like "peak", means you're comparing it to something.
Yes, we are comparing "location X's maximum" to location X's pressure some fraction of a second later or some additional wad displacement later. Well, there's nothing to compare it to--AT THAT GIVEN LOCATION.
The pressure at that location at a later time or additional wad displacement. And it's lower than the peak pressure, so it's neither peak nor maximum.
It will definitely be lower than "chamber peak" or just "peak" as we agreed. However, the pressure immediately after wad passage will also be the maximum pressure the location experiences for that shot. At any later time or additional wad displacement the pressure at that given location will be lower. And even though the pressure rises behind the wad compared to what it is in front of the wad, that comparison is really of no consequence to someone evaluating the impact of pressure on his (perhaps old and thin-walled) barrel.
Pressure differential across the wad (front to back) is not what is under discussion. What he's interested in is what's happening to the pressure in relation to other pressures developed BEHIND the wad.
At any given instant or wad displacement, the pressure is relatively uniform within the volume of gas; chamber to wad base (back to front) and across the barrel (side to side). The pressure for the whole volume varies with wad displacement and can be displayed as varying with time. Gradually decreasing or decaying, as I understand it--all the way down the barrel. Do you understand that if the wad exited the muzzle at, say, 400psi, the chamber pressure at the instant of wad exit would also be 400 psi despite a peak chamber pressure of, say, 10,500 psi? So while you're technically accurate in saying that there's a peak pressure behind the wad at any given location along the barrel, compared to the pressure in front of the wad, you're dealing with information that is of interest only from a technical standpoint--of interest perhaps to engineers, and maybe those dealing with how many angels can dance on the head of a pin--but having no practical interest to the shooter.
Not what we are talking about. The KISS principle works best when attempting to communicate with non-technical folks.
If we can get you on the same page technically as me, I expect you can help with the KISS wording- - - I'm counting on it.I think of it as I do my own results when I run a 5K road race. Let's say I ran one last weekend. Someone asks me how I did. "Well, I ran the fastest time," I reply. What I don't add is that I ran the fastest time in my age group. And since that age group is over 60, it's often the case that I ran not only the fastest time, but also the slowest time--because it was the ONLY over 60 time. How I did in comparison to the fastest runner in the entire race, or what specific time I ran, is likely what the questioner wants to know--rather than whether I picked up my standard "appearance award" for being the fastest (and "onliest") over 60 runner in the race.
Try this. Lets say you run a "race" where you are required to run as fast/hard as possible for you until you reach some set distance. You would quickly reach some peak speed and then taper off. The correlation to pressure would be how fast are you running at any given point along the raceway? If we were to plot you speed at time or distance increments, the graph would look much like a pressure trace for a firearm - IMO. 
