castnblast,
Sherman Bell did not use a "pizeo-electric device." There is no such thing.
I suspect you are thinking of piezo-electronic pressure testting, which is done by drilling a hole into the chamber wall, then inserting a piezo-eletcronic transducer into the hole--which provides a pressure reading from the wall of the case.
Bell used an electronic strain gauge, which despite being electronic is a different animal, which rather than a transducer use a stretch-sensitive sensor attached to the outside of the barrel. This can result in relative accurate pressures, but ONLY if various other factors (sensor attachment, correct measuring of chamber thickness, temperature control, etc.) are pretty precise. It can provide a relative pressure for different loads, but as a measure of absolute pressure isn't nearly as accurate as piezo measurement. (As the head of one major pressure laboratory once told me, "There are too many layers between the case and the gauge.")
Sherman Bell's results are very interesting, but hardly the last word on relative pressures with different loads in various shotguns.
