This is a good explanation taken from the CMP website:
The three gauges you will encounter are the “GO”, “NO GO” and “FIELD”. CMP only uses “GO” and “NO GO” gauges but I will describe all three.The “GO” gauge - is most commonly used when installing a new barrel and reaming the chamber to size. The bolt should fully close on the “GO” gauge, if it fully closes you can be sure you have enough room in the chamber to prevent the cartridge from being crushed during chambering. The “GO” gauge can also be thought of as a minimum safe headspace gauge and the rifle's bolt must be able to fully close with it in the chamber.
The “NO GO” gauge - is used to make sure a firearm does not have excessive headspace. The bolt should NOT fully close on the “NO GO” gauge, if the bolt cannot be closed on the “NO GO” gauge then you know your rifle does not have headspace that is excessive. The “NO GO” gauge can be thought of as a maximum headspace gauge and should not be able to fit in the rifle's chamber with the bolt fully closed. If the bolt DOES close on the “NO GO” gauge, it does not necessarily mean that the rifle is unsafe; it does however show that a further check with the “FIELD” gauge would be necessary to determine if it is safe to shoot.
The “FIELD” gauge - is used to check absolute maximum headspace. If the bolt closes fully on the “FIELD” gauge the rifle IS NOT to be fired and should be considered unsafe to shoot. CMP does not use this gauge because rifles that pass the “FIELD” check but fail the “NO GO” are approaching the point where they will be unsafe to shoot. Our standard for maximum headspace is the “NO GO” gauge to ensure our customers will be able to shoot safely for many years.
Regards,
Trapper458
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