A couple of thoughts. Yes, rotating the barrel to take readings around the circumference is vitally important. The GH I just sent back was more than thick enough...until I measured adjacent to the bottom rib. Readings became noticeably "thinner", dangerously so, as I got closer to the bottom rib and this was evident throughout most of the barrel length. Had I just measured one zone without considering eccentricity, I would have believed the gun was safe to use.
Pits. There really is no accurate and reliable way to measure the depth of a pit using a gauge. Pits can have minute pores, fissures, etc. All you can do is know that the pit is at least the depth you have measured, and know that it is likely deeper. Pits tend to erode more uniformly with fluid steels since there are no edges of dissimilar metals to follow, but until they are carefully honed out, one can never know.
This is one reason that British proof houses require that all pits be removed by boring or honing, then the bores polished, prior to reproofing. There is also the matter of the tendency of pits to grow with time and humidity, compounded by the fact that it is very difficult to get a pitted barrel truly clean.
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