Originally Posted by
stomridertx
I've adopted this method of building my uppers, but instead of using retaining compound I use stainless steel shim stock to tighten the fit of the barrel extension into the upper. If I were building a new upper today, I'd spend the money on a VLTOR MUR, BCM, or Sons upper receiver that would allow the thermal fit method. I also true the receiver with a lapping tool.
In my experience, lapping the receiver will minimize the windage adjustment on the optic or irons when zeroing. If you have an upper where you have to make an extreme windage adjustment on the rear BUIS, lapping the receiver might be the ticket for you.
I did witness firsthand an accuracy increase from tightening up the barrel extension fit to the receiver. My main and favorite AR is a BCM 16" upper with a cold hammer forged barrel purchased around 2012, before BCM adopted their thermal fit specs and were just making Milspec upper receivers. I was getting around 3 MOA accuracy with M193 ammo on this upper. As time went on and I got deeper into the hobby of putting these things together, I wanted to change the handguard out and had read about the different ways to "accurize" the AR. When I took it apart, the fit was akin to a hotdog in a hallway. I lapped the receiver with a wheeler tool and used 0.003 stainless shim stock to achieve a really tight fit of the extension. This rifle now shoots M193 1.5 to 2 MOA, and shoots Speer Gold Dot LE 62 GR .223 sub-MOA. I don't think a huge increase like this is typical, and some of that very well could be me being a better shooter now than when I first started out. I'm a full on nerd about zeroing and get the rifle really stable using bags now.