I call it the Precision Rifle smiths delight, due to it's attributes.The conventional method of headspace measuring is using a regular depth micrometer, which is accurate when it has solid support on both "wings" (lands) In the case of this old depth micrometer it sits on a small surface, that is head space gauge, and the measuring pin extends down onto the end of barrel stub,to take the measurement. The only problem here is that the Head space gauge is loosely fit in the chamber as all gunsmiths know,and can cant a little to the sides,this can give error of 0.05-0.10 mm.This "Rifle smith's Delight" is designed with a different land as the photos show. It is round and sits solidly on the barrel end and cannot cant to side,plus it's measuring depth pin contacts the head space gauge on CENTER.This tool gives constant, accurate measurements all the time.
Closer Image of Head space micrometer
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