[The Long Shot]

You should also check the gas block to barrel fit, you don't want something wiggling around affecting barrel harmonics. And check your barrel extension to receiver fit. Both of the above can be solved by some blue loctite. You can also true the receiver; Some people swear by it, but if you stick to high quality parts you should be fine.

If there is play between the upper and lower an accu wedge and o-ring won't help. Both can be compressed. I'd recommend you install the armalite NM takedown teams. If you are really after accuracy you can glass bed your upper to your lower, but that'll make it where you can only use that lower with that upper. If you have play in the front pivot pin you can get out the good ol' file, file away a bit on both sides, and use some glass bedding compound on one side of the front pivot pin area on the lower. Only do the above if you are trying to get the very last bit of accuracy out of your AR.