Shims on AR's, In or out?

There are a couple of upper shim types that I'm familiar with.
1. shim between the outer surface of the barrel extension and the inner surface of the upper receiver to tighten up the fit of barrel to receiver.
2. shim(s) between the shoulder of the barrel extension and the front face of the upper to "time" the barrel nut so that the torque range of the barrel nut results in alignment of one of the holes for the gas tube.
Headspace isn't affected in the AR system since the bolt lugs lock into the barrel extension. If the barrel extension/barrel is relocated slightly, the bolt will just follow it until locked up.
Make sense?

I'm not understanding how a shim might go *inside the upper*. That, I'd have to see. Do you mean between the barrel extension and the receiver, perhaps?

Headspace is informed by the relationship between the bolt and the barrel extension.

Not much threat of the shim affecting that if its intent is to clock the nut.

Fact is, you can check headspace holding the bare barrel in your hand - not preferred but you can - and it won't be affected by the build as long as the carrier is allowed full travel forward.

Barrel shims are the curb-feelers and antenna grounds of cars.

If one thinks they matter - they sorta do but in fact are squirrel farts and fairy dust.

I've built many hundreds of AR & M-16 uppers and never even remotely felt the need for such minute attention to the nut whether on a Service gun or Match gun.

Never had one come loose, never had one that couldn't be removed. For my part, I see a strong argument for NOT using shims on well/correctly made components.

I go out of my way to remove the residual granulation from surface treatments - I certainly don't want two more planes involved in the assembly with all that they can bring along with them.

Todd.

^^ I agree. In my previous post, I noted that I was aware of two types of shims. I, also, have not seen a need to use either one.

gun'sRgood said:

. . . but wouldn't this change head space?

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Look at an AR exploded diagram again. Headspace is set the barrel, barrel extension, and bolt. The upper, barrel nut, and any shims therein are irrelevant.

gun'sRgood said:

Hey thanks guys. I'm probably not making any sense much less explaining myself. The issue was to "time" the barrel nut to the gas tube. One guy was putting shims on the inside and the other was putting them on the outside of the upper. Still ton's to learn. Appreciate you taking an interest.

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I think you merely stumbled into a rabbit-hole of hyper-focussed vendors.

You really....... really don't need to shim there. if there was some weird extreme need for shimming, it'd be grounds for throwing one or more parts away.

Be careful to not interpret a *use* for something as a *need* for it and the torque mandated is a range, not a single targeted number.

Todd.

Shims may be needed with some Barrel nuts for hand guards. The mfg normally supplies them with the kit. On these types of hand guards there is only 4 holes locations for the gas tube to pass through. If your using a std castle nut, NO, not needed.

So YouTube is saying that to time the barrel nut you can either :
A) shim between the extension and the receiver, or
B) between the extension and the barrel nut?

I've always gone between the extension and the nut, because that is what the directions that came with my handguards said to do. But I guess either would work.

I've seen guns that it took 3 men and a witch doctor to get the nut off it was on so tight. I've also seen factory guns that the nut could be unscrewed by hand it was so loose. You can torque the nut down to a certain degree and get additional rotation, but sometimes you just need a shim.

I have never seen a standard barrel nut or a barrel nut with the 20 gas-tube holes that has not lined up with the upper somewhere between 30 & 80 ft-lbs of torque. I had one come close but it still got there before the wrench went click at 80 ft-lbs. I do have one AR that uses an Aero Precision Quantum handguard and that used shims to clock the handguard since it only has a hole for the gas tube ever 90 degrees. I also have a handguard that uses a barrel nut with no notches (AAC Square Drop) so you just torque it to 50 ft-lbs and not worried about clocking anything.

gun'sRgood said:

I'm probably just way off here but, are you saying that use torque, not shims?

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Yes.

If you look at the torque specifications as a range as intended, the notion of shims is moot.

Speaking in general terms. The range of allowable/recommended torque takes into account the possibility of 2 to as many as 5 different locators for the gas tube on the barrel nut. Taking other factors best not gotten into here.

Most folk who sell the notion of shimming take into account a single number (usually the lowest) of the allowable range of torque applications.

Todd.

gun'sRgood,

I have come across a couple of types/brands of free-float handguards that benefit from a shim (between rcvr & barrel nut) if you are not interested in CRANKING that barrel nut up to 80 to attempt to get the picatinny sections to sync.

I prefer to keep the torque on the low side, ~35, so ...

gun'sRgood said:

Ok, I've got a block for AR15 uppers to clamp onto and put some pretty good effort on it. But I can not find one for an AR10? The AR15 is a two piece that the upper fits snugly inside. So I built one out of wood. It holds everything with a pretty darn exacting fit. I made it out of white oak. It's large, gaggle and pretty ugly. But any reason not to use it?

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As long as it stands up to the torque - why not? I'd try it.

Show us a picture. I doubt that I am the only one intrigued.

Todd.

I always hone the receiver with a tool I purchased years ago and valve lapping compound and have never used a shim to replace what I've taken off,, usually torque is between 35 and 50ft lbs, I've never had to go up to 80ft lbs. I think if shims were a necessity I would have run into that.
I feel a lot of vendors hunt a fix for a non problem.

Shims should be placed between the barrel extension flange and the receiver. This position does not expose the thin shim to any shear, whereas placing the shim on the barrel in front of the flange, between the flange and the barrel nut, leaves the shim to be the bearing surface between the nut and the barrel extension.

Out of all the AR;s I have assembled, I have only had to shim one barrel nut to get the gas tube hole to properly line up and get the nut to the proper torque range. Barrel nuts are torqued to 30-80 ft/lbs. The one I had to shim would not line up to insert the gas tube in that torque range so shines were needed. I have never had to shim a GI type barrel nut or most free float hand guard barrel nuts.

Personally, I don’t shim. I use a lapping mandrel to dress back the face of the receiver thread to allow 45-50ft.lbs. torque on the nut when the tube is aligned (after seating the threads, naturally).