Dick, in addition to standard codes (how large of an indication is rejectable), there is the issue of standards to set up the inspection process - especially with UT. The instrumentation needs to be calibrated with a standard that has similar metallurgical characteristics to the barrel being tested. For all practical purposes this means the standard needs to be a damascus, twist, or what have you barrel, equivalent in metallurgical condition (chemistry, thermomechanical processing, and final heat treatment) to the barrel being tested. The standard has reference notches, flat bottomed holes or what have you that the instrumentation must "see" and detect at a specified level. Then, and only then, is the test valid. I believe there are way too many variables to conduct NDE and then make a conclusion as to the safety of any given set of barrels.

As far as proofing goes, it's been around a long while, but it isn't the be-all, end-all either. There are some barrels in the "barrel study" that show definite signs of fatigue failure - something that no single overpressure shooting event will be able to detect.