Simple question from a non-technical type: If the reading provided by the gauge depends on the dimensions of the metal, then how could a single strain gauge provide an accurate reading anywhere other than where it's placed? The strain gauge provides a trace of pressure vs time from ignition to muzzle exit. The pressure inside the barrel "combustion chamber" will be relatively the same throughout; the pressure immediately behind the wad is about the same as in the chamber. This being true, the strain gauge pressure reading at any given point of the trace will be useable as the pressure for that point in terms of displacement behind the wad. Assuming it's reading pressure in the chamber, there would not be any way to tell the pressure farther down the barrel, where the dimensions of the metal change significantly . . . would there? Yes, but we need to convert the time axis to displacement. This requires a fairly complex set of calculations; just the sort of thing computers do well. I'm not sure if any of the pressure insttruments provide an option for pressure vs displacement --- yet. As I understand, that's why Bell used several strain gauges at various positions on the barrel. This is a good approach, sure fire, if you will, but not the only approach wsith strain gauges and piezo gauges, both of which give pressure vs time traces. With crusher technology it is the only way due to the crusher giving point peak pressure only and time or displacement base data; hence the barrel bristling with crusher ports. His tests were made in an effort to determine whether the peak pressure really does vary significantly between black and smokeless, and between fast-burning and slow-burning smokeless powders. From his results--and I think also from the earlier Dupont tests--it does not appear that there is much variation at all in the location of the peak pressure (always in the chamber), although there is some in how quickly the pressure drops. But, not generally enough to reliably suggest switching from one powder to another to protect thin profile barrels.
Rocketman, you've left me in a state of confusion.
A considerably better state than knowing it all and being wrong; lets see if I can beam you up to the state of understanding. The wad moves down the barrel.
Yes, we agree on that 
. The strain gauge only measures pressure where it's placed--typically in the chamber (
yes), to measure peak pressure (
no, it measures pressure all the time from ignition to muzzle exit). Are you then saying that the peak pressure remains the same all the way down the barrel (
no)--as long as it's measured behind the wad (
the pressure behind the wad and in the chamber are nearly the same)? I don't think that's what you're trying to say, but what you are trying to say is unclear to me.
I'm saying that the trace pressure, which records the rise in pressure from ignition to peak and the decay in pressure as the wad moves down the barrel is not confined to peak chamber pressure. If you convert the time axis of the pressure trace to displacement of the wad, then you will have peak pressure at each location from chamber to muzzle. The pressure at wad passage will be peak (and then decay) for each location. If you were to locate a strain gauge half way down the barrel, it would produce a trace "identical" to the last half (based on displacement, not on time) of the chamber pressure trace. The only point Bell was exploring in his tests was whether fast vs slow powders (black included in the latter) had a significantly different pressure profile, and especially whether slow powders peaked significantly farther down the barrel than fast powders. His tests established that all powders reach their peak pressure inside the chamber. Nothing he did in that test, or attempted to do, had anything to do with selecting the correct load (which would include primer, wad, and shot charge as well as powder) to result in a pressure low enough to be within the parameters for which specific barrels were designed.
Bell explored a few other questions, but this was a main finding.
