|MadSci Network: Physics|
Pressure, which is the force exerted on a given area, is measured by changes effected on the material constituting that area, e.g. the interior surface of the gun barrel. From what you have described, I would say the subject strain gage senses the elastic expansion of the gun barrel diameter as it reacts to the changing pressures within it. The strain gage measures the initial diameter of the barrel, and then upon firing the gun, the gage measures the maxima of the diameter of the barrel as an indicative function of the maximum pressure acheived within the barrel. The actual numeric result of said pressure would have to be a function of the thickness of the barrel, the elasticity of the barrel metal, as well as its initial diameter, among other factors.
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