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.338-.408 CheyTac Ackley Improved Headspace Gauges

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Pacific Tool and Gauge offers three lengths of headspace gauges 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 gauge, the chamber will accept ammunition that is made to SAAMI’s maximum specifications. The GO gauge 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 gauge 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 gauge, 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 gauge. The NO-GO gauge 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 gauge, 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 gauges are slightly shorter than the SAAMI maximum in order to give a small safety margin.

Compatibility of Ackley Improved 40° Shoulder Cartridges: P. O. Ackley had a standard process for improving existing cartridges. He measured headspace on the corner between the chamber neck and the chamber shoulder, not at a datum lower down on the shoulder (which is what almost everybody else does). For fire-forming brass to the 40° shoulder angle, Ackley would shorten the headspace at the neck/shoulder corner by .004". That way, the chamber would interference fit (positively support) the neck-shoulder corner of the case during fire-forming. The interference fit probably also stressed the brass so it would yield more readily in the right direction when fire-forming. Thus, the GO gauge for Ackley Improved cartridges is .004" shorter than the GO gauge for the parent case, and this GO gauge isn't compatible with any other chambering. However, the standard GO gauge for the parent case, being .004" longer than the Ackley version, CAN be used as a NO-GO gauge for the Ackley Improved version of the cartridge. The shoulder angle doesn't matter, because the headspace contact is on the neck-shoulder corner, not on the shoulder as such.

For a diagram illustrating the Ackley fire-forming process, click here (Adobe PDF file).

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