The average commercial case is .002 shorter than minimum headspace, if you add a .002 shim your headspace will be at the minimum or GO.
Below is a RCBS Precision Mic for the .308/7.62
[img width=600 height=450]http://i122.photobucket.com/albums/o254/bigedp51/IMGP5130-1.jpg[/img]
The zero pictured below is minimum headspace or GO
[img width=600 height=450]http://i122.photobucket.com/albums/o254/bigedp51/IMGP5157-1.jpg[/img]
Below the Precision Mic has a new unfired Wincheater .308 case in the gauge, the gauge shows the case to be .002 shorter than minimum headspace.
[img width=600 height=450]http://i122.photobucket.com/albums/o254/bigedp51/IMGP5254.jpg[/img]
Below is a military 7.62 NATO case in the Precision Mic, military cases are .0015 longer than civilian commercial cases and approximately .0005 shorter than minimum SAAMI headspace. Military headspace is longer than commercial SAAMI headspace and the cases are slightly longer and thicker in the base web area
[img width=600 height=450]http://i122.photobucket.com/albums/o254/bigedp51/IMGP5256.jpg[/img]
To answer your question if you use a cartridge case to headspace your rifle you will be .002 UNDER minimum headspace (too tight) if you use a .002 shim you will be at minimum headspace or go.
NOTE: The act of dry firing a fired case will shorten the case approximately .001 each time you dry fire the case (drive the shoulder back) You are also warned to never use cases that were used for reduced loads again for normal pressure loads because the case will be much shorter than normal and case head separations can and will occur.
You do not need to set your headspace as tight as many of you are, the act of fire forming your cases will blow out the shoulder and make your cases a custom fit to your chamber.
I shoot and reload for the .303 British Enfield rifle, the military headspace settings are so "loose" (up to .020 head gap clearance) I put a rubber o-ring around the base of the case to hold the case against the bolt face to fire form my cases.
[img width=600 height=450]http://i122.photobucket.com/albums/o254/bigedp51/IMGP5096.jpg[/img]
[img width=600 height=450]http://i122.photobucket.com/albums/o254/bigedp51/IMGP6290.jpg[/img]
Now here is some "headspace" information to think about. (about a half inch too much)
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