Slop in fitting the barrel extension to the upper receiver can rob your rifle of its potential accuracy. AR-15 barrels don’t screw directly into the receiver. They screw into a barrel extension that slips into the receiver. Since it is a slip fit there is a lot of play. Anyone who has ever re-barreled or watched videos of folks assembling an AR-15 upper will note that up until screwing the barrel nut in place the barrel will wiggle around. Trying to correct the issue by over torqueing can damage the receiver and affect the barrel harmonics. You should apply the recommended torque and try the corrective actions we will discuss below.
This can be fixed a couple ways. You can use green Loctite, the #620 is designed to withstand high temperatures like one would expect near the chamber. The 620 was designed for slip fits where gaps are large. It is thicker than the red Loctite and seems to fill gaps between the barrel extension and the corresponding inside surface of the receiver quite well.
You can purchase some shim stock, start with very thin 0.001″ stainless shim stock. Then with shop scissors cut a shim about ¾” wide and 2 ¼” long. The barrel extension has about one inch of bearing length, not including the flange and cutting the shim stock a little on the narrow side gives you leeway to keep it centered and not extending out from either the front or back. Wrap the shim around the extension and attempt a trial fitting, first without Loctite, to see if the barrel will go into the receiver with the shim stock in place. If I still have a lot of slop I cut another replacement piece of stock a little longer. If the fit is too tight to go in place you can gradually trim the length until everything goes tightly in place. Then as above apply green Loctite #620 after making sure everything is clean and degreased.
The downside to both these methods is removal, Loctite is a semi-permanent method. You can try to remove the barrel with heat, a block of wood and a hammer but risk severely damaging your receiver, barrel or both.
The article that this paper is based on is from American Gunsmith Magazine, dated March 2013. Almost exactly 5 years ago and there is much controversy on this subject. As with all things AR, it breaks down to personal preference. Do you want a rifle to go out and plink with, or be super accurate at over 500 yards? If the later, you might want to consider shimming it up, if not, standard build methods will be just fine for you.
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