I will add in that my 9mm gauge leaves the loaded 9mm up above the rim
Hmm, out comes the barrel, well that's ok.
I had one that stuck out higher, out comes the barrel, nope, that one is not good.
Ok, if its up just a bit, ok, too high, no good.
My take is cut to spec tolerance on the gauge as it should be and the barrel is more generous to allow variation (as it should be
It a guide not a definitive. Tools are funny things, you have to learn how to use them and what is stated a lot of times has some variation to it that is not quite what they say.
I had a motor break system that had a definitive spec to set it to. I did and it would not work, hmmm.
After doing it 4 times it was, ok, its time to be Edison and experiment, I am doing this exactly right but its not working.
It wound up that it had to be looser than they said to work right and while not huge, it was significantly measurable with the scaling involved.
such is the world,