I should have given more information in my original post. If you look at precision ultra sonic wall thinkness gauges. you will see they can measure in the range we need. For example the TI-007 Precision ultra sonic wall thinkness gauge has a measurable range from .006'' to 1'' with a resolution of .001''. However if you read the manual you will see that the minimum radius for convex surfaces measured is .5'' or a little larger than a 4 bore. or 2.5'' for a concave surface. So even a 1900 dollar wall thickness gauge can not measure a barrel smaller than 4 bore.
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