True about superficial testing. But, even with them the thickness of the hard material will cause variations in the reading. You need a minimal thickness of the hard layer of .026 to check above 62RC and that is thicker than case hardening.

I have had variations of 1-2 point testing high layer count Damascus

The diamond on my Ames only makes a very tiny dent and the major load on it is 150Kg. How deep of dent does a 100Kg on the tip of a small diamond point make in the metal. You can go to the D scale and only load 100Kg on the diamond for case hardened material.

My point and Prototype Design's is that you need to do a lot of stoning or diamond filing to remove enough material to get through the hard layer in the first place. In the second place all you should ever have to do is polish of any high spots.

Barrel shanks are not hard. If you try to pen the Barrel to cylinder gap from .002 to .003 . That is just .001 with a 600 grit diamond file your going to be at it a while. If your using a 1200 grit stone and diamond plate to polish a trigger it would take you hours to get through the surface hardening on a trigger or hammer sear