I remember a recent thread in which a member found to his horror that the minimum wall thickness of a gun made for him in Europe in 1972 (?) was only 7 thou.

Assuming that no work had been done on that barrel since it was made it appears to have survived both Proof and just on 50 years of at least intermittent use without even bulging.

As there is no definite minimum thickness at which a barrel will/will not fail it is perhaps less important to worry whether the gauge is producing a reading to the nearest thou, than to regard what it shows as a “ball park” figure to show if the minimum figure is adequate, marginal or “buyer beware”.