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Checking Headspace Part II: The AR-15

By The Brownells Editorial Staff
 
GO and NO-GO gauges are used to check headspace on the AR-15, as on any rifle, but the procedure is somewhat different.
As with any rifle, a used or new-built AR-15's headspace should be checked, but the procedure is somewhat different.
 
In an earlier article, we covered the basics of checking headspace, using a typical bolt action rifle as an example. These days, the AR-15 is the most popular rifle in America, with many shooters both new and experienced discovering the fun of shooting this lightweight, reliable, versatile Jack-of-all-trades gun. Many readers might be wondering if the process for checking headspace is different for this "modern sporting rifle". Here's a brief look at how to verify your AR's headspace is safe.

CHECKING AN EXISTING GUN

As with any other rifle, you should check headspace on any used AR-15 or upper receiver you purchase from a friend, gun show, or even from a gun store. Headspace should have already been checked on brand-new guns or assembled uppers before they leave the factory, but it certainly won't hurt to verify the headspace on them.

Remove the upper receiver from the lower receiver and pull out the bolt/carrier group (BCG). Take the bolt out of the carrier and remove the extractor and ejector from the bolt. Get out the cleaning kit (#084-000-029 or #080-000-573) and thoroughly scrub the chamber, barrel extension, bolt face, and locking lugs on both the bolt and receiver extension. Excess fouling can prevent the gauge from seating properly and give you a false negative result. Reassemble the BCG without the extractor and ejector for the test.
 
Remove both the extractor and the ejector from the bolt.
Remove both the extractor and the ejector from the bolt.
 
Get the gunk out! A dirty gun can lead to inaccurate results.
Get the gunk out! A dirty gun can lead to inaccurate results.
 
Insert the GO gauge into the chamber, put the BCG back into the upper receiver, and attempt to push it into battery. The carrier should seat flush with the rear of the upper receiver. If it doesn't, the chamber might be short - or the bolt may be out of spec.

If you have an extra bolt, maybe from another gun, try it in the gun to rule out the original bolt as the problem. If neither bolt works, the chamber probably is short and should be checked by a qualified gunsmith.
 
With the upper detached from the lower, insert the GO gauge into the chamber.
With the upper detached from the lower, insert the GO gauge into the chamber.
 
IFlush fit: it's a "GO" if the carrier seats flush with the rear of the upper.
Flush fit: it's a "GO" if the carrier seats flush with the rear of the upper.
 
One warning: A brand-new BCG from the factory will have a Parkerized finish or other coating on it that could tighten the dimensions just enough ("tolerance stacking") to cause a false negative result with the GO gauge. This is almost never a problem with a used gun because any finish wears off quickly after a few rounds are fired.
 
To avoid a false negative result, it's best if the Parkerizing has been removed from the locking lugs on bolt and barrel extension.
To avoid a false negative result, it's best if the Parkerizing has been removed from the locking lugs on bolt and barrel extension.
 
Now, remove the BCG and GO gauge and insert the NO-GO gauge in the chamber. Repeat the test. This time the carrier should extend beyond the rear of the upper receiver. If it seats flush, headspace is excessive. If your AR-15 closes on a NO-GO gauge, take it to a gunsmith to correct the headspace before you shoot it.

Otherwise, reassemble your rifle (don't forget to reinstall the extractor and ejector on the bolt), take it to the range - and happy shooting!

Here's the AR-15 variation of the GO/NO-GO gauge "magic formula": BCG flush with receiver on GO + protruding on NO-GO = Safe To Shoot

CHECKING HEADSPACE WHILE BUILDING A RIFLE

If you're in the process of building a new AR, you can check headspace before you install the barrel in the upper receiver. Again, remove the firing pin and ejector from the bolt. Insert the GO gauge into the chamber, followed by the stripped bolt. Try to turn the bolt right and left. It should turn in both directions with slight to moderate pressure.
 
With the gauge in the chamber, the bolt should easily turn in the receiver extension. This cut-away barrel shows how the gauge seats inside the chamber.
With the gauge in the chamber, the bolt should easily turn in the receiver extension. This cut-away barrel shows how the gauge seats inside the chamber.
 
If the bolt begins to turn but you feel resistance, the chamber is probably at minimum headspace dimensions. If the bolt won't turn at all, it may be out of spec or the chamber may be too short. Try a different bolt; if it turns, the problem is the first bolt, not the chamber. If the new bolt also won't turn, you have a short chamber. Next stop: your favorite gunsmith's shop.

If your AR-15 chamber "passes go," get your NO-GO gauge and insert it, followed by the bolt, into the chamber. The locking lugs should not engage, and the bolt should not turn. If the bolt begins to turn but stops before the lugs fully engage, the chamber is likely still within SAAMI specs. Again, try a different bolt to see if the original bolt is the culprit. And if you have any concerns, have the rifle checked out by your gunsmith.

SPECIAL NOTE FOR COLT-MANUFACTURED AR-15s

Colt's specification for maximum headspace exceeds that of a standard NO-GO gauge and needs to be tested with a special 5.56 maximum headspace gauge (#319-418-033). Rifles that close on a standard NO-GO gauge but do not close on this Colt gauge are considered "serviceable" for military use. However, if you reload your own ammo, be prepared to get fewer reloads per case from ammo fired in one of these guns. The military often operates their weapons in adverse conditions and does not re-use their spent brass, so for them it's not a problem. For the civilian shooter who reloads, this "long" chamber will result in excessive "working" of the brass, leading to case head separation, neck splitting, and overall shorter brass life.
 
With safe headspace properly verified, your freshly-built or "new" used AR-15 is ready to shoot.
With safe headspace properly verified, your freshly-built or "new" used AR-15 is ready to shoot.
 
Editor's Note: For a "hands-on" demonstration of the concepts discussed in this article, check out these Brownells Tech Tip videos, "What is Headspace?" and "AR-15 Headspace".