This is from Forster's website:
Forster offers three lengths of headspace gages per rifle caliber. In order from the shortest to longest, they are: GO, NO-GO and FIELD:
1.GO: Corresponds to the minimum chamber dimensions. If a rifle closes on a GO gage, the chamber will accept ammunition that is made to SAAMI’s maximum specifications. The GO gage is essential for checking a newly-reamed chamber in order to ensure a tight, accurate and safe chamber that will accept SAAMI maximum ammo. Although the GO gage is necessary for a gunsmith or armorer, it usually has fewer applications for the collector or surplus firearms purchaser.
2.NO-GO: Corresponds to the maximum headspace Forster recommends for gunsmiths chambering new, bolt action rifles. This is NOT a SAAMI-maximum measurement. If a rifle closes on a NO-GO gage, it may still be within SAAMI specifications or it may have excessive headspace. To determine if there is excessive headspace, the chamber should then be checked with a FIELD gage. The NO-GO gage is a valuable tool for checking a newly-reamed chamber in order to ensure a tight and accurate chamber.
3.FIELD: Corresponds to the longest safe headspace. If a rifle closes on a FIELD gage, its chamber is dangerously close to, or longer than, SAAMI’s specified maximum chamber size. If chamber headspace is excessive, the gun should be taken out of service until it has been inspected and repaired by a competent gunsmith. FIELD gages are slightly shorter than the SAAMI maximum in order to give a small safety margin.
As I understand it:
The GO gauge corresponds to SAAMI's minimum headspace dimension, the NO-GO gauge isn't an actual SAAMI dimension but rather an intermediate length that the gauge maker determined that a properly chambered rifle should conform to. The FIELD gauge corresponds to the SAAMI max headspace dimension and if the gun will chamber the FIELD gauge then the chamber is bigger than SAAMI's max and considered unsafe. GO minus .004" sounds about right for NO-GO. GO minus .010" should correspond to FIELD. I agree that .010" is too much, but it's still safe and the case shouldn't rupture if fired in a chamber that's over sized by that amount. It's not real good, but it's not going to come apart on you.