What roalco said. A mechanical engineer with interest in NDT looked into the $1900 TI-007 Precision ultrasonic wall thickness gauge, which has a measurable range from .006" to 1" with a resolution of .001". However, the manual states 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.

And BTW I affixed a tape measure on the in-the-barrel arm of my Bill Henry gauge which makes confirming the location/depth much easier




https://sites.google.com/a/damascusknowledge.com/www/home