There is a lot to be said for lapping the upper receiver face to the barrel extension. You don't want to touch the inner diameter of the upper receiver though, you want that snug. But that front face is never perpendicular to the bore on the mass produced AR. With a lapping tool you can make it so.
But the biggest thing is the barrel. A good quality barrel is the heart and soul of an accurate AR. I've got 2 Black Hole Weaponry barrels and they shoot better than I can. 1 is a 5.56 and the other a 6.5 Grendel and they will both stack bullets on top of each other. I am sure other barrel makers can do the same, just look around and you will find one.
But the biggest thing is the barrel. A good quality barrel is the heart and soul of an accurate AR. I've got 2 Black Hole Weaponry barrels and they shoot better than I can. 1 is a 5.56 and the other a 6.5 Grendel and they will both stack bullets on top of each other. I am sure other barrel makers can do the same, just look around and you will find one.
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