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Dimensions and Tolerances of Manson Headspace Gauges:
Confusion often arises when comparing headspace readings generated by gauges from different manufacturers–a rifle may accept both “GO” and “NO GO” gauges from one maker but only the “GO” from another. This would lead a reasonable person to ask, “why?” The answer lies in the dimensions and tolerances to which the gauges are made.
It’s important to understand that the various makers of headspace gauges do not use the same dimensions for what is termed “GO”, “NO GO” and “Field”. We know the specifications used in the manufacture of our gauges, but cannot speak to those used by other makers–except in very general terms—so the information given here applies only to Manson-manufactured headspace gauges.
We use both a two-gauge and three-gauge system for checking headspace. Three gauges (“GO”, “NO GO”, and “Field”) are used with rimless, shouldered rifle calibers, eg. 30-06. This is done because SAAMI allows .010” between minimum and maximum headspace for these chamberings. Many feel (ourselves included) that only knowing headspace within .010” is imprecise and that an intermediate gauge is called for.
In rimless, shouldered calibers, our “GO” gauge corresponds to the SAAMI (or industry) minimum; “NO GO” is ground to minimum plus .006”; “Field” corresponds to SAAMI maximum (min. plus .010”). Our manufacturing tolerances are such that actual gauge measurement will always fall within SAAMI limits.
We offer two gauges as standard on all other calibers—shotguns, rimfire, rimmed, belted or rimless pistol(45 ACP). In this system, “GO” corresponds to the SAAMI (or industry) minimum and “NO GO” to SAAMI maximum. Again, tolerances are such that actual gauge measurement will fall within SAAMI limits.
We do make headspace gauges on special order to specifications other than those outlined above. All such gauges are clearly marked and are easily differentiated from our standard product.
Please contact Manson Precision Reamers if you have further questions concerning our headspace gauges or any of our many other products.