If someone wanted to make a really, really nice gage for measuring wall thickness, there are woven phenolic materials (rods) available that are super rigid and would possibly suffer little to no flex when used. Further, a notch could be cut on the end of the rod that would allow various screw-in indicator tips to be used instead of a ball on the end which would allow for very accurate measurements of pits or other irregularities in the bores.
A gage made like that would be a bit spendy, however as they say, the right tool for the job is priceless. Here's a picture of various tips that could be used on the end of the rod. I think tips like these are what Brian's referring to..
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Threaded Mode