The .38 line of cartridges started life in the rimfire era using outside lubricated heel bullets which were actually a nominal .38" in size. With the desire for inside lubricated bullets, bullet diameter was dropped to fit inside the case & a hollow base was used to expand & fill the rifling, thus was born the .357" diameter bullet. The .38 S&W began life with an inside lubricated bullet, but apparently simply played upon the popularity of the "38" name. The .38 WCF was the same as .38-40 & also used the popular name of 38 as they were a nominal
.40 caliber. The .38-55 Ballard used as I recall a bullet of about .378" diameter so is a nominal .38 caliber if bullet size rather than bore size is used.
There are plenty of other examples, including the .22's you mentioned, .22, .220, .218, .219, .224 & etc.
I spent 35+ years as a machinist so I have used all these methods of measuring. You will get close with the small hole gauge in the bore with an odd number of grooves. The biggest problem would likely be if trying to check a size against the stamped marking, you might read it a bit oversize. If the bore marked 40 was close to the upper limit it might read as a 39 size.
Not a big deal, but would not prove the proof house's mark to be wrong.
