With most head space gauges, the bolt must be disasembed, and the ejector removed to do the test. With Foster Head Space Gauges, there is no need to take the bolt apart.
If you ever have your barrel or bolt replaced, you will need to have the Head Space checked. If there is not enough, the bolt will not be able to fully close on the breach and bullet. If too much head space, you could have a explosion, with parts of the bolt, shell casing, and receiver being sent flying. Making sure the Head Space is correct, and a good bolt will prevent this. These test are done with a clean bolt and chamber, the operating rod and slid disconnected, and lightly, only using 1 finger, slide the bolt and gauge forward with the gauge going into the chamber. The fitment test is seeing if the bolt closes, and rotates fully with only your finger providing the movement.
With a new barrel, a Go Gauge is used to insure the chamber is the correct size. This is the only gauge that the bolt must fully close on. If you have not had the barrel replaced, there is no need to do the Go Test.
The next test, is the Field Gauge. With it properly installed, and it not fully closing the bolt lug against the receiver, the gun is Safe to Shoot. If you are installing a bolt from another gun, and it passes the Field Gauge test, make sure to retest it a few times after shooting it some to allow the new surfaces to wear in.
The most stringent gauge, is the No Go Gauge. If the bolt does not fully close on this with just your finger, the gun is safe, and has a long life ahead. Passing the No Go test equals a gun with a barrel/bolt combination equal to a new one, with many years of life in it.
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