Ar accuracy problems

[fmyellowj]

The only thing I have ever done (to all of my AR upper receivers) is square up the fave of the upper receiver. I have a friend that owns an Upper Receiver Lapping Tool that I borrow when I'm doing a new build.

Receiver lapping is something I also do regardless of upper or barrel manufacturer. I also use a little but of Blue Locktite between the barrel extension and the upper receiver, not on the threads but where the extension meets the upper.
Is the gas block installed straight? I've heard that can affect accuracy. Other than that I'd check to make sure the barrel wasn't contacting the handguard during firing, assuming it's free floated.