Yes, Dewey he is true that ultrasonic measure is far better than any other method, not to mention no one has access to it.
In the real world, mechanical devices are used to measure, some with bad accuracy, some with good, and some with very good, depending on quality construction, but specially on the hands that use the device and the conditions.
I took the device on Sunday to a friend, is is a master mechanic, taking the responsibility of installing and certifying huge power pumps around the world for nuclear plants, and similar things. He has been in the desert at 50 degree, changing a 1 ton shaft from a water pump, verifying bearings, run outs, etc. He knows his business and nothing was done without his signature and personal measurement and check. Of course, millions of dollars are involved in those business.
He has also been gunsmith
He said the truth, the vertical device is not exact, but in the correct hands it is almost as perfect as we need. He also mentioned no measure is perfect unless perfect conditions are used, and if several measures are done, they must me done in exactly the same conditions. That is why in the desert, they always made measures exactly at the same time, for example at 6am, and not at 4pm after the sun has expanded all the metal.
Also he mentioned, that in the metrology/measurement world, ranges exist.
So yes, I already knew the measure I made might have a very slight variation, but it gives a very approximative photo of the condition of my barrel, and for me, that is better than nothing, more than just looking at a bulb and saying how shinny and correct the barrel is as many people do....
