Pressure is a function of force and area, either static or dynamic. There are three types of pressure, 1)absolute, 2)differential, 3)gauge. Of these three, which would best describe the pressure created in a shotgun barrel once a shell is fired? Then there are different types of gauges to measure pressure, strain, etc. Having said that, if one wanted to measure psi for their own reloads, how would such a guage be constructed? Thanks in advance.