I have never used one to measure shotgun barrels.

They are commonly in use in industrial piping environments.

Because, when things flow through a pipe, they wear it out from the inside, leading to leaks, big, and small.
So, they are pretty common.

I’ve seen two types, one type connected to a computer screen, and one type just touched on the surface.

On the computer screen one, the operator move the transducer back-and-forth, and identified very precisely where the voids were behind the surface.

What I am trying to say, is that the people using the tool are not super geniuses, but the tools work very well, so the learning curve isn’t long, and doesn’t require complex fixturing or technique.

Out there doing it best I can.