For over a century, "headspace" has been what SAAMI's glossary says:
Quote:
HEADSPACE
The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.
HEADSPACE GAGE
A device used in a firearm to determine the distance between the breech face and the chamber surface on which the cartridge seats. Also called Breeching Plug.
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For rimless cases, it is the distance from the breech face (bolt face in a bolt action rifle) to the chamber point where the case mouth stops against when the firing pin falls.
For rimless bottleneck cases, it is the distance from the breech face (bolt face in a bolt action rifle) to the chamber shoulder reference point where the case shoulder stops against when the firing pin falls.
For belted cases, it is the distance from the breech face (bolt face in a bolt action rifle) to the chamber point where the case belt stops against when the firing pin falls.
For rimmed cases, both straight or bottleneck, it is the distance from the breech face (bolt face in a bolt action rifle) to the chamber point where the case rim stops against when the firing pin falls.
A GO gauge is marked with the dimension that's minimim for SAAMI standards. The NO-GO gauge is marked with the dimension that's SAAMI spec maximum.
Headspace has never officially meant to be the space between the breech face and the case head of a round full forward in the chamber, but it's far too often misused this way. It's correctely called head clearance as stated in SAAMI's glossary:
Quote:
HEAD CLEARANCE
The distance between the head of a fully seated cartridge or shell and the face of the breech bolt when the action is in the closed position. Commonly confused with headspace.
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Headspace for many rifle cartridges can be seen at
http://www.saami.org/specifications_...wnload/206.pdf