I tried searching but didn’t find much. I’ve built quite a few uppers and never had to true any of them. My last upper build has been giving me fits. My rails don’t want to line up properly, and the barrel isn’t quite centered in the rail (left-to-right). I got it pretty darn close, but upon shooting it I had some excessive windage issues. I tried a different receiver, different rails, and soon a different barrel. I’m using quality parts (BCM, Rainier, Centurion, Troy, Fortis, etc) so I’m not sure why I’m having these issues. I bought the receiver truing tool from Brownells and I have some lapping compound on the way. Has anyone used this tool and what has your experience been? I never thought it was necessary in the past but this build has me thinking otherwise.
Nothing that lapping compound with do is going to fix whatever problem you’re having here. What exactly is “excessive windage”? How many clicks off of mechanical zero?
Which hand guards and upper receivers have you tried?
I can’t remember how many clicks from mechanical zero it was, around 16 - 20 I think. I have some adjustment left but it bugs me as I prefer my rear sight to be as close to center as possible. I’ve tried a BCM (new one) and Rainier receiver, and for rails I’ve tried Centurion 12.0, Fortis Rev 14.0, and Troy Alpha 12". The Troy seemed to work the best FWIW, probably since it doesn’t clamp around the nut like the other two. I figured truing the receiver will at least rule that out as the problem. A couple thousandths at the receiver translates to a fair amout of error at the end of a 12" or 14" rail I would think.
Have you tried a different barrel nut? I don’t know if the Fortis or Troy use their own nuts, but If you’re running all this on the same standard Barrel nut, that’d be my next area of interest.
Yes, tried 2 different barrel nuts. All three rails use a standard nut. I have a used Colt nut I’m going to try next.
I read somewhere that there was an issue with an AR-15 that the point of impact would change greatly as it warmed up. It was driving the owner crazy.
He finally decided to try truing the front of the upper receiver, he found that it was not square to the receiver bore. Upon truing the receiver and reassembling the upper it now stayed at the same POI whether it was hot or cold.
I now use the Brownells tool on all of my uppers, you would be amazed at how out of square some are.
I can believe it based on the discussion of BCM’s upper receiver and the amounts of uppers rejected by Bravo in that thread.
Has anyone found a VLTOR MUR1 series to be out of square yet?
Tried a new DD barrel along with the Colt barrel nut I had. Currently set up with the Rainier upper and the Fortis REV 14. The barrel looks dead center in the rail, which is good, but the rail still doesn’t sit right. It is lower than the receiver and trails off at a slight upward angle. I feel prety confident that the windage will be improved, but I’d really love it if the rail sat level. I’m leaning toward lapping the receiver to see if it will help. Here is a pic to show what I’m trying to correct:
Did you use a acog mount or some other mount to bridge the gap before tightening the rail?
Yes, I’ve been using a carry handle to line everything up, however once I tighten the screws down I end up with what you see in the pic. The Centurion is even worse. The Troy fits pretty well, but I’m planning to use that one on another build.
I would imagine truing the receiver face would not help as the barrel surface sounds square to the receiver bore axis, but the threads sound like they are not concentric which not matter house square the face, the barrel nuts will always tilt up. If all of your rails tilt up when mounting them
I’ve used the Brownells truing tool a bunch. What I find works best is using a Geissele reaction rod when torquing the barrel nut and the KAC Picatinny rail vise block when tightening the rail to the upper (it keeps the receiver and rail in alignment). Upper receiver quality has a lot to do with windage. Some have the index pin notch way too loose or bored too big where the barrel extension mounts. Some the receiver threaded end isn’t square and/or I’ve seen the bore of the receiver isn’t straight in alignment within the upper (you can see this typically at the back end of the receiver, one side will be much thicker than the other). I remember a few years ago that the flat top rail on a RRA receiver that was about 3 degrees canted.
I mocked up the rail on a receiver with just a barrel nut and no barrel, thus removing the receiver face from the equation. It looks much better in terms of alignment. Short of both my barrels being out of whack, I think the receiver face is the only thing left. Thoughts?
That definately makes it look like the thread are good and concentric, is that with the nut torqued or close to the same position if a barrel was on and the rail torqued to the nut?
Also, do you have another barrel you could try? it could be the barrel extension face causing the issues compared to the receiver face.
I just positioned the nut roughly where it would be if there was a barrel in place. I did tighten the rail onto the nut. I mocked it up this way with the Centurion as well, and it also lined up nicely. I originally built the upper with a Rainier Mountain barrel, but I’ve now tried a DD barrel and I’m having the same issue.
All the lapping in the world isn’t going to fix the problem illustrated in post #9. Were the clamp bolts tight when you shot the picture in post 14?
Agreed. Those just flat DON’T line up at all.
Yes, I tightened the rail onto the nut for the pics in post 14. I received my new Odin Works barrel today so I might try putting it on an upper to see if there is any change.
I wish this and the thread in precision semi auto would merge. I keep getting the two cornfused!