The British Proof houses use plug gauges. That's good enough for me. As to those who wish to use other methods, then that's down to them .
As to those who question the accuracy due to variation in cutter size or due to wear ,that's why there are tolerances with maximum and minimum's both in diameter and depth.
Shot gun cartridges are not accuratly sized like metalic ammunition so the tolerances are much greater.From memory a 12bore chamber has .010"in diameter and about .100" in depth.