AR15 Shim - Nut or Barrel?

[cuervo]

I was reviewing various AR15 videos, and saw that many of the uppers were being built with shims going over the chamber-end of the barrel and then being slide into the receiver. This would be used to slightly extend the barrel out of the receiver, affecting the alignment of the notches in the barrel nut for the gas tube.

Another installation video showed shims going over the muzzle end of the barrel and staying between the receiver and the barrel nut. The purpose was the same: slightly set back the barrel nut so that the gas tube could be aligned correctly. However, the barrel is fully in the upper receiver.

So,
1. How does the first method not affect head space?
2. Is one method better than the other?
3. Is this at all dependent on the rest of the system being used, e.g. M-Lok vs GI hand guards?

 

[edwardware]

1. Find the barrel lugs, then find the bolt lugs, and you'll notice the receiver isn't related to headspace.

As for 2. and 3. . . depends why you're shimming. There's no reason to on a standard AR, so you must be aligning or tweaking something outside the standard design.

 

[cuervo]

1. Maybe not headspace, but it would seem that moving the barrel farther away from the bolt would affect something.

2/3. I had an AR upper that came with shims under the barrel nut for an M-Lok hand guard. While shooting, I noticed my RDS was canted, and realized the nut was loose. When I worked on it, I ended up removing the shims to get alignment and proper torque, but it did make me wonder why the shims were under the nut and not inside the receiver.

 

[shoobe01]

No idea what this shimming is but important part of understanding the AR-15 (or really: most self loading rifles designed since the 1950s or so) that many don't know (so sorry if anyone thinks this is too basic) is that it doesn't have the old school problems of headspacing because locking recesses are integral with the barrel.


Or rather, the barrel /assembly/. Barrels are just barrels, with threads on the back end. "Threads?!?!" you say, no the receiver has threads and a nut comes on from outside. That attaches the barrel assembly. The barrel itself has threads, and then they put a Barrel Extension on the end which has the locking recesses in it (without this assembly it would be very hard to cut the locking recesses anyway. It is reasonably carefully set up and other things about the design of the system are held to safe tolerances so mostly it's plug and play. Moving the barrel around in the receiver has NO impact on the headspace.

If by some miracle you stretched out or broke the locking recesses, you change the barrel. On an HK roller lock or FAL or Garand/Kalashnakov... if you do that, it's generally complex receiver change time stuff.

(If you want to get serious about headspace for match accuracy purposes, they totally sell matched barrel extension / bolt combos.)

 

[12Bravo20]

The torque value for a standard mil spec barrel nut is from 30 foot pounds to 85 foot pounds. This gives a wide range to allow for the barrel nut to align with the gas tube. So shims are normally not needed when using a standard barrel nut.

Now some free float handguards do require shims to get the nut and handguards times and lined up with the receiver. In this case I always put the shims between the barrel nut and the receiver. I know a few companies that supply shims with their free float handguards for this reason and they go between the barrel nut and receiver.

I have never used shims that go between the barrel extension and receiver.

 

[powermad]

Just follow the instructions with the handguard you use.
I've had two handguards that the barrel nut needs to be timed, didn't need to shim either one but it said too if needed.
I'm not buying anymore like them though and am swapping one out.

 

[Varminterror]

1. How does the first method not affect head space?

Shimming between the receiver and barrel extension flange does not influence the relationship between the bolt and extension. In a manner of speaking, the bolt has a lot of float in how far fore-and-aft it can move relative to the upper receiver.

2. Is one method better than the other?

I prefer to shim between receiver and extension, as there is less opportunity for the shims to get damaged by the torque applied to the barrel nut. Effectively, sandwiching between the receiver and extension flange puts the shim in a static position, whereas putting the shim on the forward side of the flange between the extension flange and the barrel nut means we’re exposing the thin little shim to all of the shear force of the nut as it is tightened, meaning it has opportunity to wrinkle or bulge out of position, or split.

3. Is this at all dependent on the rest of the system being used, e.g. M-Lok vs GI hand guards?

Not inherently, no. Shimming is an issue for aligning the barrel nut, which can be applicable for any barrel nut which requires timing for the handguard and/or gas tube.

Personally, I prefer to lap the receiver face to adjust these dimensions rather than stacking shims. It takes a little longer, but it’s more appropriate, as it doesn’t introduce some bandaid solution into a high compression, high torque situation.

 

[cuervo]

Thanks for the replies.
The upper I had came assembled. I think it was loose because of the shims (or maybe just laziness). Once I removed them, I torqued the nut a good bit greater than 30ft-lb, but I know I stayed below 80, and was able to get things lined up.

 

[Charlie98]

I prefer to shim between receiver and extension

How do you get around the receiver pin?

 

[gpb]

When I needed a shim to correctly time the barrel nut for passage of the gas tube and to obtain in spec torque, I cut the shim to get around the barrel index pin.

See this you-tube clip

 

[shoobe01]

For a minute there I was also baffled, how you can cut a shim to fit around things. Forgot that in that position nothing spins and my brain was very much still locked into round-shims-spin . Seems simple now, good to have video to get my brain around it at least though.

 

[Varminterror]

How do you get around the receiver pin?

Cut the shim. It’s a static application on that side of the flange, no issue from cutting to move past the the indexing pin.