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How does one measure that critical bolt-chamber distance ?
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It's pretty easy really. You can either use a no-go gage like ptg makes--or take a piece of sized brass and add thin spacers to the rim face until you you can't get the bolt to close without significant force--record what that measurement is. You should remove the ejector and extractor to get a precise measurement. As I've said, if that total space can conceivably exceed case support and still be within SAAMI max spec (which in the case of my Faxon AR set-up it could with starline brass)--toss of the dice whether your cartridge has necessary head support.
The thing is--the ptg no-go gage is set to not close at 1.714--so AR's and other auto-loaders might be compliant with SAAMI specs and still close on the 1.714 no-go gage. Not good.
I've already decided I'm going to use nothing but Hornady brass since it's the only brass that comes at a max length of 1.71. I wish I had known that before I ordered the 1,000 or so starline and winchester cases.