Here is some proof of the flexibility of the "traditional" gauge construction. The following video shows just how much movement there is in a .625" diameter water-hardening steel rod, at 16 inches unsupported length, chucked in a 5C collet in the lathe. This setup is much more rigid than any baseplate mount. The amount of deflection (a lot) versus the force applied (very little) should illustrate why ten different people will get ten different readings of the same barrel.
"But it's used in a vertical position" you say. Think about the fact that half of the barrel length is above the uppermost end of the arbor and that the inner wall of the barrel being measured MUST contact the arbor. Do you really believe that you can hold the inner wall in contact with the arbor, while not inducing ANY side load and causing it to deflect at all?
"But it's better than nothing" you say. No, erroneous information is worth exactly nothing.
"But so-and-so uses it and always has" you say. Of course, it looks snazzy, it impresses those that don't know any better and it's cheap to make. As you'd expect, it is also monumentally overpriced to buy, which further cements its "credibility" with the purchaser.
Play the video at half speed for a better look.
Sadly, it is the best of the mechanical measuring methods available. Yet more "tradition" that is best forgotten.