Roalco is correct.
The $1900 TI-007 Precision ultrasonic wall thickness gauge 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.
https://www.abqindustrial.net/store/inde...2ab880033fe5556