This is what I have learned about headspace and I thought it would be helpful to newbies.
When you are checking the headspace on a M1 Carbine you are actually checking the chamber where the bullet casing sits when it is chambered in the gun. The headspace gauges measure for excessive or minimum tolerances for the bullet casing. If the headspace is excessive or too small it could let hot gases escape the chamber violently and damage your firearm and you. There are three headspace gauges generally used to measure headspace in M1 Carbines. They are . 1. "GO" Gauge 2. "No Go" Gauge 3. "Field" Gauge
The "Go" gauge is used when a new barrel has been installed on a M1 Carbine. It checks to see that there is a minimum amount of space in the chamber for the .30 Caliber Carbine round to seat in the chamber. The bolt should close on the "Go" gauge. If it doesn't the chamber will have to be reemed until there is enough room for the bolt to close.
The "No Go" gauge checks for a factory specification maximum. This is an optimum measurement and is how the headspace is measured in the factory when the Carbines are produced. The bolt should not close on a "No Go" gauge. If it does check your rifle with a "Field" gauge.
The "Field" gauge measures a maximum headspace for weapons that have been fired. The bolt should not close on a "Field" gauge. If it does you could have a serious problem. Have a gunsmith check the rifle or switch bolts until you find one that passes the "Field" gauge headspace test.
Here are a few photos of me checking the headspace of one of my M1 Carbines. Photo 1 shows a "Field" and "No Go" gauges. . Photo 2 shows what the bolt looks like when closed on an empty chamber. . Photo 3 shows how to put the gauge in the chamber. Be sure the wide end of the guide goes into the chamber. . Photo 4 shows that the gauge has been pushed about half the way into the chamber. . Photo 5 shows me slowly letting the bolt move forward. The narrow end of the gauge should rest against the face of the bolt. I push up on the gauge with my finger through the mag well as I slowly let the bolt close. "DO Not" let the bolt slam the gauge into the chamber.The gauges are machined to exacting tolerances and can be damaged by this. The chamber might be damaged too. . Photo 6 shows that the bolt did not quite close on the "Field" gauge. Compare it to Photo 2. If the bolt closes on a "No Go" gauge check it with a "Field" gauge. The bolt should "NOT" close on the "Field" gauge. Do not fire your M1 Carbine if it does !!!If it doesn't close you are good to go..... . . I hope this helps because the concept of checking headspace has confused me for years. Please point out any mistakes I have made in this brief guide and I will change it. Thanks.........