This is true. The gauge checks the shoulder position, but unless you have a really tight chamber, if it fits the gauge it will most likely fit your rifle. You would have to have an oversized (Diameter) sizer to push the shoulder back enough and be too fat to chamber.
Not necessarily. I have a full length sizer and a 223 Remington rifle that must be at opposite extremes of the tolerance ranges.
Certain cases resized in that particular full length sizer will not chamber in that particular rifle but will chamber in other 223 Remington rifles.
But, the errant cases will slip nicely into my Dillon case gauge.
The die does not size the lower body of the case enough. It has nothing to do with upsetting the shoulder.
I have another full length sizer that i do not have the problems with. All cases resized in it fit any of my 223 Remington rifles.
Since the gauge manufacturers say their gauges are cut generously in the diameter dimensions, in my opinion, it is not reasonable to expect the gauge to simulate a chamber in the diameter dimensions. Obviously, if a case will not fit a case gauge, it will not chamber, but, in my experience, it can fit the chamber gauge and not chamber.
While this may seem like a rare occurrence and take a certain set of circumstances, I feel this kind of combination may be one of the reasons folks feel they need for small base dies with 223 Remington AR rifles. Not the only reason, but one of the reasons.
Without having several 223 Remington rifles, two full length resizers, and a small base resizer, I never would have discovered the true reason for cases not chambering in one of my rifles. I started my investigation because I was chided on one of the forums that my dies were set incorrectly which I knew was not the case. My preconceived notion was that I absolutely needed small base dies and I set out to prove that. To my surprise, that was not the case.