One eye-opening opinion of Dewey was in regard to the mechanical measurement of wall thickness. In his opinion, current favorite devices are inaccurate because the internal arm of the device is affected by gravity deflection. He relies on an electronic device that emits some sort of signal (sonic? xray? radio? ) through the metal and the return signal somehow triggers an electronic readout. The device is more expensive than the purely mechanical devices. The device is used in industrial applications. Gil
Much of the potential inaccuracy of mechanical barrel wall thickness gauges stems from having the unit set up horizontally. In this orientation, gravity can have the greatest effect of creating a slight deflection of the internal rod, due to lateral forces applied by both gravity and the operator. This is why many guys mount them vertically. Another source of inaccuracy comes from operator error. Not zeroing the dial or digital indicator, moving along too fast, not measuring all possible areas, can all provide false or inaccurate readings. The measuring end must be exactly perpendicular to the axis of the bore, as any deviation will give a larger than actual measurement.
There are several methods of NDT (Non-Destructive Testing) that could provide measurements of tube wall thickness. The most common used in industry is Ultrasonic NDT, and I'd guess that method might be the most affordable to a well equipped gunsmith or machine shop. There are several other methods of NDT testing of tubes that could provide wall thickness measurements including Eddy Current NDT, and radiographic X-ray gauges. I've worked on Ultrasonic NDT measuring equipment, and IMS X-Ray gauges numerous times, but don't pretend to be an expert on them. I can definitely say that X-Ray IMS gauges are not going to be found in any gunsmith shop because of the enormous cost, and the fact that they use a radioactive isotope source such as Cesium 137. Naturally, they can be dangerous to untrained personnel, and they are strictly regulated by the State and Federal Government. I always used a Geiger Counter to confirm that the shutters were closed on the Cesium 137 sources, and in particular on one occasion when a proximity switch for shutter closed position had failed on one source unit, and had to be replaced.
From what I do know about Ultrasonic NDT, it would have the same problem as mechanical units with wall measurements on double barrels in all areas between the barrels, and under the ribs. Fortunately, those areas are least likely to be externally struck thin during manufacture or refinishing. But there would be no way to measure the depth of pitting in those areas. And I would think that Ultrasonic NDT would not be at all useful for Damascus barrels due to the fact that they contain literally hundreds of inches of welds running between the iron and steel layers. So it would be very difficult or impossible to interpret the transducer feedback coming back from all of those welds within the barrel walls, at all different depths. I suppose a large pit or inclusion might still stand out from all that noise though, but that's just an educated guess.