A lot of people wonder what exactly headspace is and if it is necessary to check in their firearm. To give you a better understanding, here is a basic run through of what headspace is, the danger of excessive and insufficient headspace, and headspace gauges.
Headspace
Headspace is the distance between the face of the bolt and the point that stops forward motion of the cartridge in or at the chamber. A headspace gauge measures this clearance. The gauge is designed to resemble the cartridge for the chamber you are checking. For that reason you need to have a set of gauges that is specific to the round your rifle shoots.
When to Check
There seems to be a never ending debate on when you should check your rifles headspace and no one clear answer. The majority recommends that you should do a headspace check with a newly assembled firearm, and every time you replace a bolt, slide, or barrel. For the most part a newly bought AR 15 has probably already been checked if it was test fired. Also, if you are a hand loader you should routinely check headspace every thousand rounds or so because the continued use of ammunition that causes too much pressure can increase headspace.
Dangers of Headspace
If headspace is less than minimum the action may not properly close, factory loaded cartridges may not fully chamber, and it may not fire. If the rifle does fire you may have excess pressure that can be dangerous to the shooter, or possibly cause damage to the locking system. On the flip side, if headspace exceeds the maximum limit you can have problems like failure to fire, misfires, disrupted accuracy, and excessive case stretching which could lead to the case rupturing.
Headspace Gauges
GO Gauge
The GO gauge is considered the minimum safe headspace gauge. A rifle’s bolt should be able to fully close with a GO gauge in the chamber. The GO gauge is most commonly used when installing a new barrel and reaming the chamber to size to ensure a tight, safe, and accurate chamber.
NO GO Gauge
The NO GO gauge is used to determine if a firearm has excessive headspace. Unlike the GO gauge, the bolt should NOT fully close with a NO GO gauge. If your bolt cannot fully close then you know that your rifle does not have excessive headspace. If your bolt does close with the NO GO gauge it would be best to consult a gunsmith. Just because it closes doesn’t necessarily mean it is unsafe, and you can do a further FIELD gauge test to determine if it is absolutely unsafe to shoot.
FIELD Gauge
The FIELD gauge is used to check absolute maximum headspace. If the bolt fully closes with a FIELD gauge inside, the rifle should be considered unsafe to shoot. The Civilian Marksmanship Program doesn’t use the FIELD check because rifles that fail NO GO and pass FIELD are approaching the point that they will be unsafe to shoot.
For most gunsmiths the standard for maximum headspace is the NO GO to ensure the rifle will be safe to shoot for years to come.
Hopefully this article can be a good reference for you when referring to headspace and the different types of gauges.