Headspace for a firearm is the distance from the datum point within the chamber to the bolt face. That datum point for a regular bottleneck cartridge is the midpoint of the shoulder. For a belted mag rifle it is the bolt face to the ridge where the belt sits. On handguns it is different. The 45acp for example headspaces off the case mouth as does the 38 super, 9mmL, and many other semi-auto cartridges. That is why when loading a 45acp you can have problems if you over crimp the case mouth. It may allow the cartridge to go too far into the barrel ending in either e CLICK or a BOOM OH SOB MFER because it just blew your hand off.
I'm not Sheldon! I'm the Flash! And now I'm going to the Grand Canyon to scream in frustration *takes two quick steps* I'm back