This is a short excerpt from “Gunsmith Tools, Cutters & Gauges – A Primer”.
Pictured here: “Forster’s Datum Dial, can be used to check headspace gauges or ammo for correct headspace.
Reloaders often set the headspace of the ammo wrong, either making it hard for the gun to lock up properly, or creating excessive headspace by bumping the shoulder back. This tool makes it easy to check ammo and prove to the client an error.
Note: the indicator reads .0035″ this is the difference between the two gauges in the picture. Well within tolerances.”
This method is measuring from a diameter along the shoulder to the head of the gauge. Overall length of the gauge has NOTHING to do with headspace when talking about rimless bottle neck cartridges.
You would be amazed how often this information has to be imparted to people who are learning about headspace. We provide this tidbit of valuable information here to save to from silly errors that make life much harder than it really is. Check out the complete book.