How to Check Headspace on an AR-15
Woody's answers are based on published federal and state law. Not legal advice. Consult a licensed attorney for your specific situation.
Use AR-15 headspace gauges in GO and NO-GO lengths. Insert GO gauge - bolt should close. Insert NO-GO gauge - bolt should NOT close. This verifies proper chamber depth.
How This Affects Your Build
Headspace determines how far the cartridge sits in the chamber when the bolt closes. Incorrect headspace can cause dangerous pressure spikes, case head separations, or feeding failures. Most factory AR-15 barrels come with proper headspace, but it must be verified when building or after barrel changes.
The Detail
Headspace checking requires dedicated gauges made to precise SAAMI specifications for 5.56/.223 chambers. The GO gauge represents the minimum acceptable headspace, while the NO-GO gauge represents the maximum safe headspace. You'll also want a FIELD gauge for worn chambers, which represents the absolute maximum before replacement becomes necessary.
To check headspace, remove the ejector and extractor from your bolt to prevent interference with the gauges. Insert the GO gauge into the chamber and attempt to close the bolt. The bolt should close completely with normal effort. If it won't close on a GO gauge, the headspace is too tight and the barrel extension may need to be lapped or the barrel replaced.
Next, insert the NO-GO gauge and attempt to close the bolt. The bolt should NOT close completely on this gauge. If the bolt closes on a NO-GO gauge, the headspace is excessive and potentially dangerous. This indicates the barrel extension wasn't properly torqued during manufacturing or has been damaged. Some gunsmiths also check with a FIELD gauge, which should never allow bolt closure in a serviceable rifle.
When checking headspace, ensure your upper receiver and barrel are properly assembled with the barrel nut torqued to specification. Loose barrel nuts or improperly seated barrel extensions will give false readings. The bolt carrier group should be clean and the bolt should move freely without binding.
Build Impact
Most quality barrel manufacturers headspace their barrels correctly from the factory, but verification is essential for safety and reliability. If you encounter headspace issues, the barrel typically needs replacement rather than attempting field fixes.