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VEry interesting and handy for future reference. I have no doubts that you are measuring correctly since you are experienced in those kinds of matters.
When I first broke down and finally bought the fancy Hornady gauge, I also had a few surprises that ran counter to my intuition. Notably, I noticed that when using FL sizing dies to simply re-size the neck without trying to bump the shoulder back, the shoulder datum actually grew.
But that's not your case I realize. When I measure the head space on the shoulder I get variations of only 0.001" or so. And that is with different brands of brass. The only thing I can even guess about that is different from your situation to mine is that I currently have been either FL sizing or partial full length sizing (properly now) after every firing and loading. I wonder if the effect you are seeing is a result of variable loads and variable numbers of firings and only doing neck-sizing each time.
Yeah, theoretically, when you fire a round in your chamber, the brass expands to completely fill the chamber. And then it shrinks back a tiny bit each time. But I am wondering if after 3 firings, does it necessarily shrink back exactly the same amount as after 1 firing? Or after 7 firings? I know brass does become brittle after awhile, so the spring-back will not be as much. And it stands to reason that hotter loads will have a different effect than mild loads.
You say you went ahead and FL sized the 7mm cases? Presumably, they all measured the same after that? It will be interesting to see if they all measure the same after firing this time.
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