I might be missing the point. My thoughts on gauges where that they defined a dimension by acceptance or rejection. How does something adjustable define anything but what the user wants.
At one time, long ago, I made thread and taper ring and plug gauges. They were all inspected by an independent source and certified to meet standards. So one day I walk by a machinist who was cutting internal threads(I think it was a class 3 4.5" 8tpi if my mind serves me well). He was filing on the male thread gauge. I asked him what he was doing and he said that he had fed in enough and the gauge should fit. I looked at his insert and it was broken off. Head in hands I walked away.
Since you are probably a hand loader and 223 AI is not an off the shelf cartridge it probably doesn't mean anything to be exact. But what is it you are trying to do that keeps you from setting the headspace from a sized piece of 223 AI brass?
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