I would go back and check the upper receiver barrel socket to make sure it and its front face are on center line of the upper receiver, as well as check the barrel extension to make sure that it was on center line of the barrel.  

Also a quick check of the barrel extension to the upper socket to make sure that it was seated correctly during the install.


Upper barrel socket is quick work to check that, and even true that since I have a upper Mandel for the lathe, as well as just putting the barrel between two live centers to check the run out of the barrel extension and even if the barrel is bent.

Could have bent canted the upper receiver barrel socket using too much pressure to set the barrel nut to the correct torque and index.


If you did not crack the barrel socket to the receiver, but just cant bent the barrel socket to the barrel center line, it can be fixed.

Myself, I have a mandrel that is OD step sized to fit both the upper receiver main channel and the upper barrel channel tightly (has to pressed in and out of the upper receiver) that will not only true the barrel socket to the center line of the receiver, but also tweak out the upper receiver main channel if someone vise blocked up the upper without the center insert and squeezed the channel walls slightly inward.   From there, just chuck the upper with mandrel in a lathe using a live center on the front end of the mandrel, and make a few light passes to square up the front barrel socket edge to the center line of the receiver as well.





Also, since I will have the barrel off the upper and in hand as well, just chuck it up in the lathe with some live centers both ends to check the barrel and barrel extension to make sure they are running true.  Plus, allows me to clean up a questionable barrel crown as well.