I'd like to thank the Preacher for his link to Dewey's Blog. I also recall that the Preacher has, in the past, also posted photos of his own Bill Henry dial indicator tool being used in a horizontal orientation.
Most folks seem to understand and agree that this type of tool is less prone to error or false readings when used vertically.
Flexing of the round support bar does not seem to be relevant to the relationship between the pin on the bar and the pin of the gauge, unless I contact the barrel wall with the bar.
Since the pin on the bar is pressed or screwed into the bar, it stands to reason that any undue lateral pressure on that pin will cause some slight deflection of that bar. Dewey didn't just make an unfounded statement concerning that potential error. He actually provided a detailed explanation, along with a short video showing just how much a tool steel bar can and will deflect when a lateral force was applied. He even used a trigger gauge to show us just how much force was applied to cause the deflection seen on his dial indicator.
Talk is cheap. Dewey backed up his talk with graphic video evidence. I didn't need such evidence to prove what I already know, because I have had to use a steady-rest on my own lathe when turning longer pieces of round steel stock, to prevent deflection during turning. I have also seen long steel line shafts or mandrel bars, up to 10 inches in diameter, flex SEVERAL INCHES under their own weight when lifted by a crane at their mid-point. I didn't need any dial indicator to see that long solid steel bars are quite flexible. The flexible nature of steel is also seen in seemingly rigid torsion bars.
I even worked with a guy who lost his left testicle when he unwisely decided to spin a stainless stainless steel rod in a 3600 rpm three jaw chuck polisher. The unsupported rod abruptly flexed enough to partially castrate him as soon as he applied lateral pressure with a piece of emery cloth. Ouch! Glad I didn't get to see that one.
It is also common knowledge that if you gave five guys five micrometers, or dial, or digital vernier calipers, and they measured the exact same item, they would likely get several slightly different measurements. This is why Machinist and Tool & Die Apprentices are taught proper measuring technique to minimize errors. I remember my High School Shop Class teacher showing us how the same measuring tool could give very different readings when used improperly. That potential operator induced measuring error is compounded with a trickier setup, such as trying to measure barrel wall thickness down deep in a shotgun barrel by using a long measuring instrument that can and will flex slightly under various lateral pressures.
That said, I would agree with those who feel that despite the known potential for error, a careful and skilled hand could use one of these tools to obtain reasonably accurate measurements. And let's face facts... the majority of shotgun barrels are not honed dangerously thin. So a slight measuring error is unlikely to put someone into a dangerous situation. Of course, no matter what sort of tool is used to measure barrel wall thickness, calibration before use is very important. And should readings on the thin side be found, especially at the points where pressure is still high, and eyeballs or fingers are in close proximity, then extreme caution, and/or a qualified second opinion should be sought before using the gun.
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.
Actually, the Ultrasonic Measuring equipment is rarely used to measure pipes or tubing after it wears out. That wear is more commonly detected when it begins to leak or abruptly ruptures. Then it is simply scrapped and replaced. But it is very commonly used in industry to measure wall thickness and find defects before the new product is shipped to the customer. My own experience with Ultrasonic and other Non-Destructive Testing (NDT) equipment was mentioned on page 3 of the "Wiggle in the Barrels" Thread. I have repaired them and/or made logic changes or edits in the PLC programs, but never worked as an Ultrasonic NDT operator. Companies wishing to cover their butts and avoid liability spend big money to purchase and maintain NDT testing equipment, so I assume they are better than the older and less efficient mechanical measuring systems.https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=614531&page=3
I guess my comments in response to GLS in that Thread, and Ed Good's subsequent request, is what prompted the Preacher to start this Thread.