Determine Correct Headspace
Gives the proper chamber headspace dimensions so you know a firearm is safe to fire. GO gauge gives the SAMMI-recommended, minimum chamber depth. NO/GO is slightly shorter than the SAMMI-recommended to give a small safety margin maximum chamber depth. Precision-ground and hardened to provide long service life.
MIXING AND MATCHING BRANDS OF HEADSPACE GAUGES - Don't do it!! That's the short answer, here's why. It boils down to tolerance stacking. Each Headspace Gauge manufacturer works within a range specified by SAAMI. Manufacturer "A" may work at the high end of the range while Manufacturer "B's" gauges are in the middle of the tolerance range. Mixing the two could give an inaccurate reading. By sticking with one brand within a particular caliber, you will eliminate a variable. If you have a Forster, .308 GO gauge, get a Forster, .308 NO GO. Use Clymer with Clymer and Manson Precision with Manson Precision, etc. You can use any brand of headspace gauge with any other brand of chambering reamer. e.g. Forster Headspace Gauges with Clymer Reamers, Manson Precision Gauges with JGS Reamers, etc.
Big Book Catalog, Issue:70, Page:457
SPECS: M7 high speed steel, in-the-white, hardened.