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. 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 point of the trace will be useable as the pressure for any point 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.
