Fguffey- Phil Sharp identified headspace as the distance between the cartridge head and the face of the bolt. He also said it could be called clearance meaning between the cartridge and bolt. He predated SSAMI I believe, and all the people I knew growing up, and still those I know now, understand for the reloader, headspace is the measurement of the distance between the bolt face and the cartridge head, most usually discussed for our reloaded rounds.
I can understand Phil Sharp writing a book about something he never saw before. Before SAMMI there were head space gages, they were the same as SAAMI gages with one exception. In the beginning they did not use 'the datum' they used the case body/shoulder juncture to the bolt face. It should not have been confusing because the old gages were stamped with the length of the chamber from the case body/shoulder juncture to the head of the gage. It should not have been confusing for a few because the shoulder, shoulder/neck juncture and mouth of the case were located in the correct place.

I understand reloader believe I am calling them stupid; you understand it is impossible to fix stupid. Phil Sharp would have been doing reloaders a favor if he had said "I do not know". Reloaders have been trying to reinvent reloading by throwing 'head space' into ever conversation. Not me, there is something else besides 'head space' for example by design the length of the chamber is go-gage length, the case is said to be minimum length. If you are a fan of Phil Sharp the length of the case from the datum to the case head is not .005" shorter than a go-gage. And then we advance to go-gage length, after go gage length there is no such thing as field reject length.

And then there is my M1917 with a chamber that is .002" longer than a field reject length chamber, I understand there is no interest but a minimum length case has when fired in my M1917 has .011" clearance. And then I have to start over because of fans of Phil Sharp because he has convinced you and all the rest of his fans the clearance is between the bolt face and case head. The clearance on all of my M1917s is between the shoulder of the case and shoulder of the chamber.

Head space and case head separation: When I fire 30/06 ammo in one of my M1917 there is no stretch between the case head and case body. I know, it should not be necessary to explain that but no one understands the shoulder does not move and if Phil Sharp understood that we would not have so many confused reloaders.

F. Guffey