I am trying to install my Atlas M5 handguard barrel nut on a new Aero upper and I can't get the nut timing correct for the gas tube.  At 30 ft lbs the nut tube cutout doesn't line up with the tube hole in the receiver, and as I incrementally increase the torque all the way to 80 ft lbs it still doesn't line up.  I still have quite a ways to go, I wouldn't be surprised it I need to take it well over 100 ft lbs to line up.  I don't want to damage the aluminum receiver threads and I already exceeded what Aero says to in their instructions (65 ft lbs).

There are shims in the handguard box, I tried those between the barrel nut and the barrel shoulder but the nut just chewed up the shims which are so thin they have the rigidity of aluminum foil.  

I saw this video on youtube where the guy places the shims between the barrel extension and the receiver.  

https://www.youtube.com/watch?v=QZe5cQ5Ir5E

I think that would work better to keep the shims intact but it seems odd to me and I wonder if it could create problems.

Ive put together a few 223 uppers before and never needed shims, but this 308 aero nut has alot fewer tube cutouts so the torque range between cutouts is higher.   Right now I'm thinking about calling Aero and see if I can get another barrel nut that will hopefully time okay.  

Any ideas?

Never put the shims between the barrel extension and the receiver. Many people lap the face of the barrel hub to ensure that it's flat and true, but if done improperly, it will ruin the upper. That can help with timing, but I wouldn't count on it. Check for burs and imperfections on the barrel flange and the nut shoulder. Make sure the barrel nut face doesn't contact the receiver. Normally you season the threads by torqueing to 35 ft-lbs three times, but you probably already have that covered. Use Aeroshell 33-ms or synthetic moly grease on the receiver threads. I keep the grease off the shims, but put a very light coat of thin oil (#9) on the shims to help keep them from bunching up. The shims don't like to be torqued on and off, so figure out how many you need and try to do it once. Buy extra shims if needed, they're cheap, but shipping hurts. I usually hand tight the nut and shims so it's clocked around 11:30, which usually gets me where I need to be when torqued.

I was using a light coating on lithium grease on the receiver threads and seasoned the threads a few time by torquing on and off, that may have helped some but still not enough.  No burs that I could see.  I checked the backside of the nut to make sure it was not making contact with the receiver, had a 20 thou gap so I was good there and without the barrel the nut would run all the way down and contact the receiver so I knew the nut was bottoming out only on the barrel extension.  

I only had two new shims left in the package since I ruined the first 3.  Fortunately one of the two new shims was the thickest (5 thou) and fairly rigid.  I decided to place the shim deep inside the barrel nut instead of placing the shims on the barrel first, this way the shim never made contact with the nut threads.  The first go around I placed the shims on the barrel first and then ran the nut over them, so each thread of the nut had to pass over the shims before things tightened up, leaving lots of opportunity for the threads to catch on the shim and bugger it up.  At 65 ft lbs the nut tube cutout lined up with the receiver tube hole so all is good now.    

I am fairly certain that once the nut comes off it will catch on the shim and ruin it, seem to be a single use design, so I just ordered a few packs of shims from Aero in case I ever need to do this again.  Shipping cost more than the shims

Thanks for the advice!

I put the shims between the barrel extension flange and the upper receiver.  I cut the shim so that i can get it past the indexing pin.  I put the cut behind the indexing pin.  this way there's no torque exerted on the shims.  no problems.  on both AR-10's and AR-15's.

Bison Armory barrel shims

Bison Armory Shim Set Instruction Video


TBH, I've seen different instructions for these shims recommending between the barrel nut and barrel extension, and also as posted above, between the barrel extension and receiver.  IMHO, it makes more sense to go between the barrel extension and the receiver, to eliminate the torque twisting action on these fragile precision shims.  So, that's what I do.

You can do your own evaluation and choose however your rationale leads you to conclude.

I went down the youtube rabbit hole again and found Aero made a video on how to install the Atlas handguard.  In the video they show sliding the shim over the barrel first and then tightening down the nut, this is exactly what I did the first go around except that half way in I heard "crunch crunch" as the nut ate my flimsy shims (1, 2, and 3 thou).

Aero video

This other video shows what I ended up doing to try and prevent the nut from eating the shims (look at the 3:29 mark).  He put the shim all the way down inside the nut against the shoulder.  For me, this prevented the shim from getting eaten by the nut threads as it screws over the shims and onto the receiver threads.

https://www.youtube.com/watch?v=oyF9wzxs2C0

If I needed to use one of the thinner shims instead of the 5 thou I used I don't know if this method would work very well since those 1-3 thou shims are so delicate the twisting action of the nut against the extension may just tear it and cause it to bunch up.  I could see the advantage here of placing the shim between the barrel extension and the receiver instead.  

I am starting to like the idea of placing the shims between the receiver and the extension, I am trying to think of any downsides to that but can't think of any.  At least tearing up the shims is not a concern, and the shim would likely be reusable.

Other than possible headspace/timing issues (granted, we’re talking about thousandths of an inch), I can’t think of any downsides either.

delete

Between the extension and receiver is the proper location.
I think you misspoke.

Placing a shim under the nut allows the rotational force to tear or bunch the shim. This may create an uneven surface and a false torque.
Shims are never placed directly under a nut.

The bolt head spaces off the inside of the barrel extension. There is no chance of changing the headspace

Perhaps headspace was the wrong term. By placing the shim between the barrel extension and the receiver, you change the distance from the face of the chamber to the face of the bolt. The shim is pushing the barrel forward because you’ve added material between the 2 surfaces. Again, we’re talking about thousandths of an inch.

Aero themselves say to place the shim between the barrel extension flange and the barrel nut.

...

The position or travel of the bolt, has nothing to do with the receiver.
The space between the head of the bolt and the chamber would only be changed if you installed the shim on the face of the bolt.
Your logic is incorrect.

Never mind…….

If the shim is placed between the receiver extension flange and the face of the upper receiver, what happens with the index pin and the corresponding slot in the upper receiver when the barrel is installed into the receiver?

it still goes into the slot no problem.

you do realize we're talking about shims that are thousandths-of-an-inch thick?  do you have any feeler gauges?  try pull out the one that's 0.003" thick.  not the 0.030", but the 0.003".  feel that, that's how much we're talking about.

In all my time installing handguards, I don’t ever remember seeing “cut the shim” in the manufacturer’s installation instructions. Must’ve missed it...every time. Granted, I don’t have experience with every handguard out there, but if you know of a manufacturer that instructs you to cut the shim as part of the installation I’d love to hear about it.

You guys do you, no sweat off my back. But I’ll continue to follow the installation instructions provided by the people that designed and manufactured the handguard.