You need access to the inside and outside of a barrel to measure its thickness, unless you have a rather complex X ray instrument that can take slices. The barrel thickness inside the rib presents the same problem to the barrel wall thickness gauge as it does to the simple micrometer.

A .015 off center would show up when you are micing the outside, if were only at one point. To be .015 off and not picked up by the outside mic, the barrel has to be bored .015 off center the length of the barrel. Since we are usually investigating guns that nearly 100 years old it difficult for me to imagine a barrel wall initially being .020 and not have failed by now.