I'm in the process of a building a lightweight rifle, I went to install the barrel in my upper last night and noticed that the barrel extension did not fit snug in the upper receiver. Once I aligned the index pin and seated the barrel in the upper, there is still a very slight wiggle that can be felt between the barrel and the upper receiver. I have built a few uppers in the past and they all required some finesse to get the barrel extenstion seated all the way in the upper. I did not try it with Aeroshell on the extension and upper yet. Is this normal for there to be some movement before the barrel nut is torqued down or is it possible that my upper receiver is out of spec? Both parts are from quality manufacturers but the upper is a blem due to a misaligned logo. Thanks guys!

TL;DR:  yes, a little wobble is fine.

The longer story:  The index pin serves to keep the lugs oriented properly so that the bolt can freely engage and lock with the barrel extension.  There are tolerance allowances so that a little left-right wobble won't interfere and the pieces will still mate up fine.  The first Rifle I ever built was with a basically free barrel that was missing that indexing pin.  I tried using a clipped piece of coathanger, doing it without, etc, but the torque from the nut would always cause the rotation to exceed what was allowable.  Basically the bolt wouldn't pull out.  Eventually I got it to work right with a small cut nail that was the right size.  Now the rifle is at over 10k rds with 2 malfunctions in its life.

If you're concerned it may be too much wobble, just put the barrel in and push it to the "extremes" and dry cycle the BCG.  If the BCG can freely lock and unlock with the lugs on the barrel extension, it's good.  If it won't then contact the manufacturer and they should be able to take care of it.  Hope this helps.

It depends on your intended use for the weapon, for most purposes probably ok.
However if by wiggle you mean the barrel can rock when pressed against the upper, then you may have a high spot on the face of the upper which can cause issues.

Torque the barrel nut to between 30 and 80 lb-ft of torque and the "wiggle" will be gone.

It's possible that the reciever needs to be lapped to the barrel, although as previously stated a little wiggle is fine.

Some people would go to the extreme of lapping the face of the receiver, and even "bedding" the extension into it. However, as others have mentioned, the wobble will go away with a properly torqued barrel nut.

Have you tried adding a barrel shim?

aluminum receiver (soft) meets steel barrel flange (hard). once torqued properly the wobble should be gone.


as others have said, dont be scared to inspect the receiver and make sure the front is free of any deformations prior to putting the two together.

(I'd also recommend installing the barrel into the receiver using a vice with a set of barrel blocks, NOT a receiver clam shell)

Wobble is normal, but contrary to other opinions I will say it's not "fine". Service Rifle match shooters learned a long time ago that a tight extension-to-receiver fit is critical to accuracy. It may feel right when you torque the nut, but it will still wobble enough to affect accuracy even if you can't see/feel the wobble. I wouldn't sweat it you are building a truck gun, but if you're going to scope of and shoot prairie dogs I'd address it. Shim it or bed it. Personally, I don't worry about it on my CQC builds, but my CMP match upper had wobble so I bedded it (it now reliably shoots 0.7 MOA).