So, in my opinion, fundamentally, “headspace” is a measure of how much of the case head is sticking out of the chamber.
I could have worded this better.
I would have to disagree with that assessment.
Unsupported case head has nothing in the world to do with headspace.
That is one strong statement, no equivocating about that, unsupported case heads (case head protrusion) have nothing, absolutely nothing, nada, zlich, zero, to do with headspace.
I would have to disagree with that assessment. Decades ago minds more brilliant than any here came up with a system so simple that a trained monkey could assemble, maintain, safety check, a firearm, given that the monkey used the parts, tools, gages, and followed the procedures provided to him. The SAAMI headspace definition is a good one, but leaves out a lot of assumptions that the monkeys don’t need to know as long as they do what they are told without thinking much about it. But, the monkeys are interpreting the world based on the gages and sizing dies they have without any overarching understanding of what is going on.
In rifles, the thickness of the extractor and the rim of the bolt head have more to do with how much case has to stick out of the barrel than anything else and they are unrelated to headspace. Different firearms designs will have more or less unsupported case head but identical headspace.
SAAMI spec only specifies the headspace dimension as a distance forward of the bolt face. Differences in the thickness of the boltface rim and/or extractor would effect how much of the case is unsupported by the barrel but this has no effect on headspace whatsoever.
This reads as though case head protrusion is similar to the question “where are the cows in the pasture”. It is not something you can really specify with any exactness as in a couple of minutes the cows will be someplace else, and 10 minutes hence they will be will be in a random location and the whys and wherefores unintelligible to the human mind. So, is cartridge protrusion sort of like where the cows are in the pasture?: random, no exactness, a metaphysical concept ?