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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:
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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