A "Go" gauge is the gauge that is used to measure minimum headspace. When you place the "Go" gauge in the rifle and close the bolt, the bolt should close and lock easily and completely.

If the bolt does close successfully then this tells you that the firearm has at least sufficient headspace. It may still have excessive headspace, but that is determined by the "No-Go" gauge.

If the bolt will not close on the "Go" gauge then you may not have sufficient headspace. This means you do not have enough headspace to chamber a cartridge properly and can damage the rifle if you force the bolt to close.

1. Clean the face of the chamber. There may be unseen gunk and dirt at the face of the chamber. I have owned many Mosin-Nagant rifle and carbines that at first were very difficult to chamber a cartridge. Only later did I discover (with the help of a flashlight) that there was gunk and dirt fused to the face of the chamber. Gunk and dirt that is so dark and hardened that at a casual glance, and without the aid of a very direct and bright light, it can be easily overlooked. After it is successfully cleaned and/or removed then it is quite possible you have eliminated the headspace problem and the "Go" gauge check should be repeated. If the same results occur then.....go to the second fix (below).
2. Take it to a gunsmith. You need to have the rifle looked at by a gunsmith and the rifle will need to be modified or adjusted (possibly a chamber reaming, replace bolt parts, or moving the chamber face of the barrel closer to the bolt.) in some way to compensate for the lack of headspace.

"No-Go" gauge

A "No-Go" gauge is the gauge that is used to measure the maximum allowed headspace. When you place the "No-Go" gauge in the rifle and close the bolt, the bolt should not close and lock. If the rifle does close on the "No-Go" gauge you may have a problem that needs to be tended to by a gunsmith and may be an unsafe weapon to fire.

"Field" gauge

If the rifle fails the "No-Go" gauge then you should measure using a "Field" gauge. A "Field" gauge is used to measure the largest possible safe headspace dimensions. When you place the "Field" gauge in the rifle and close the bolt, the bolt absolutely should not close and lock. If the rifle does close on the "Field" gauge you have a problem that needs to be tended to by a gunsmith and an unsafe weapon to fire.

If the bolt closes on a "No-Go" gauge but does not close on the "Field" gauge this means you have a rifle that has excessive headspace but is safe to shoot. Basically it will stretch the heck out of brass and good luck trying to reload