On the AR the bolt is removed and disassembled. The head of the bolt has the pins driven out and the ejector claw and the ejector cylinder pin removed leaving a striped bolt head. The striped basic bolt is temporally put back together and closed on the "GO" gunge and it should close. The bolt is removed from the receiver and the "No Go" gauge is inserted the bolt is brought forward but this time it should not close. The gun passed the test. If the gun will not close on the "GO" gauge it has a head space problem! If you shot the gun it may not close on a loaded round and you can have the bolt driven down your thought. If the gun closed on the No Go Gauge and you shoot the gun it can blow up.
Companies that make the gauges Forster, another Clymer,
Graff and Sons sell Head Space Gauges for about $5 to $10 under price of many others pricing and as much as $25 under others. www.grafs.com No I do not have any connection with this company
Richard W. Norman Author of American Handbook on Guns Ammo and Freedom