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 longer than the SAMMI-recommended to identify excessive chamber depth and an unsafe condition. 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 ra Mfg: Manson Precision SPECS: M7 high speed steel, in-the-white, hardened.