A note here,
When I was an armorer in the Marine Corps,
the manual stated (and still does) to torque the barrel to 30 foot/pounds and then "tighten to align."
What they're talking about, is lining up the Star nut so that the gas tube will pass through it and into the upper receiver without binding.
The only problem with this is, occasionally, you have to tighten the barrel to 70 foot/pounds or more to get the star nut to line up!
Oddly, and for reasons that neither I nor the "best minds" at Marine Corps Logistics Base Albany Georgia couldn't figure out, was that this very often caused the rifle to shoot as much as 42 inches left at 200 yards!
It never affected right windage or elevation, but it would OFTEN send the round left of target.
The fix that I came up with, was to tighten the barrel to NO MORE THAN 45 foot/pounds and then if there is a point of the star nut in the way of the Upper Receiver Gas Tube Hole, very carefully dremel the offending point off of the nut.
Of course, this is an unauthorize modification in the Corps, but I've done it to several personal AR15s and it works like a charm.
No ill side effects noticed after thousands of rounds!
Hope it helps!
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Your mind is your primary weapon.
USE IT!