AR15 Barrel nut requirees excessive torque

[Awesomesauce]

Calgunners, just wanted to share an experience that may save some time and frustration in the future.
I was putting tgether two uppers this afternoon, and one of the barrel nuts was just not aligning properly. It was perfectly aligned at 20 ftlbs, but I hit 80 ftlbs before clearing the next notch. I did this a few times but it was not getting better (was trying to get thread stretch from repeated torque).
I was looking up shimming (.001 inch) the barrel to get a better mechanical position for gas tube alignment. In my searching I found a post on ARFCOM that suggested that a super careful sanding of the reciever face would also work. The explaination was excellent and I quote it below:

DO not removeTOO much. the amount is only about .001" to .0015". thousandths.

I use 400 to 600 grit. You only have to remove about .001" of anodizing to get it down to around 80 ft lbs. It is an extremely small amount that has to be removed. Generally not even all the way through the anodizing. A whole notch on the barrel nut is only .0027". It is easy to remove to much and be right back where you were if you are not careful.

Thread is 1.25 x18 threads per inch. 1/18 is .05555" for a full turn of the barrel nut. There are 20 notches on the nut. .05555/20 is only .0027777" for a full notch of movement. You do not want a full notch. At .001" thousandths there will be over 1/3rd of a notch in movement.

I used a small piece of 1000 grit wet/dry sandpaper and a glasstop table to ensure a flat surface. I laid the sandpaper on the tabletop and held the reciever face flat on it, working four directions and in a circular pattern. As I cut into the anodizing layer, there was a light purple residue on the paper. I checked periodically to ensure I did not cut through the anodizing completely. This process worked well, and I was able to fit the barrel nut on at 65ftlbs, when before it was at 80 lbs with 1/4 notch to go.

[liber]

That is exactly why most vendors include shims. I have 2 Parallax Tactical FFSSRs and they provide shims to use. The recommended torque is 30-80 ft. lbs. Using 2 shims on a 5.56 I got it lined up perfectly with 45 ft. lbs.

[Awesomesauce]

True, however if you are in the situation without shims this is a solution as well. I did not even cut through the anodizing, just a very mild hone to the surface.

[croc4]

I have found that squaring up the upper fixes this issue, plus it ensures the barrel seats square. Brownells sells the tool.

croc4

[milotrain]

I'd be really careful sanding the face of the reciever. You really want to keep things perpendicular to the action. Ideally you use Brownell's tool or put it in a lathe. At worst I'd glue your sand paper to a piece of glass and use a framing square to keep it straight, then use very light strokes so you don't round it over.

[ar15barrels]