Q. How can one determine if their AR 15 rifle has a NATO chamber or a 223 Rem chamber when it is not stamped on the barrel? I have your headspace gages including the NATO 556 Max gage. The bolt will close easily on the Go gage and will close on the NoGo gage, however the bolt is tight. It will not close on the NATO Max gage.
A. Checking the headspace of your rifle will not give you the information necessary to determine if your rifle’s maker intended it to be a 223 or a 5.56 Nato chambering.
Please note that the 223 and 5.56 chambers have identical headspace lengths of their chambers. This is in contrast to the 308 and 7.62 chambers that do have different minimum and different maximum lengths for headspace of their chambers.
It sounds as if the checks you have done on the chamber of the rifle confirm that the rifle will definitely accept ammo of maximum size (the Go Gage closes). It sounds as if the headspace dimension is just about .0035 or .004” (three and a half to four thousandths) longer than minimum acceptable headspace (the NoGo check). That is why the bolt was tight when it did close on the NoGo gage. Finally, it has checked out as having a safe condition with the 5.56 Max gage. The 5.56 Maximum gage definitely should not close in your rifle and it did not. The headspace of your rifle is in tolerance.
You could get an 11 gage set from us to more closely determine the actual headspace of your rifle. The extravagance of owning an 11 gage set is probably is not necessary as you have all three gages you should need to monitor safe usage of your rifle.
Posted in: Headspace Gages FAQs