AR barrel shimming

[bmiller]

Purchased a Faxon gunner barrel. I really like the barrel, and how it handles in the gun. I am having a problem with a wandering zero. I am using a seekins triangular handguard. I am also using an older m&p upper. The barrel extension did not feel super tight sliding into the upper. Could shimming the barrel fix my problem? The barrel seems to shoot well, but the groups wander. The picture is a group I shot prone with a block of wood as a brace under the forearm at 200 yards. Problem is it is not where I was aiming! Does anyone have experience with this problem?

Thanks

[DRichardson]

What optic and mount are you using?

[bmiller]

Vortex 1x6, larue mount. Never had an issue in the past, did the same thing with my t-1 mounted.

[mpom]

What torque setting are you using for barrel nut? Acceptabkerange is 35 to 85 foot pounds, i believe. Some folks use a sleeve retaining compound such as loctite 609 to bed the barrel extension in upper. Would try that for a loose fit. Needs to be heat resistant and not permanent. If play is excessive, say over 10 thousands, i would replace the upper as new ones are around $100.

[josh2600]

...

Edited June 7, 2015 by josh2600

[sschultz]

bmiller

When I put my rifle together I measued the barrel extension and the inside of the upper then subtracted the the numbers to find the difference. I bought a piece of shim stock and put the barrel in the freezer overnight then put a heat gun on the upper added the shim and slid the parts together. It made a for a good sloid connection.

[bmiller]

bmiller

When I put my rifle together I measued the barrel extension and the inside of the upper then subtracted the the numbers to find the difference. I bought a piece of shim stock and put the barrel in the freezer overnight then put a heat gun on the upper added the shim and slid the parts together. It made a for a good sloid connection.

Thanks, I will give this a try

[co-exprs]

My 308 came shimmed from JP. I reused the same shim when I installed the new barrel.

[mpom]

Are we all talking about the same thing?

Placing a shim between the barrel extension flange and front of upper receiver is not uncommon, useful for timing the barrel nut, so gas tube or handguard end up in correct position, all while remaining within a reasonable torque setting for barrel nut. This has nothing to do with taking up excessive space between the barrel extension and oversized hole in upper receiver, which I think the OP was referring to.

[bmiller]

Yes that is what I was referring to

[TennJeep1618]

Some folks use a sleeve retaining compound such as loctite 609 to bed the barrel extension in upper. Would try that for a loose fit. Needs to be heat resistant and not permanent.

I've done this on a couple AR's with good results. Basically bedding the barrel into the upper.

[Graham Smith]

I ran into a similar issue with my first AR and found that I hadn't tightened the barrel nut down enough. Turned out that the problem was that the threads on the upper were just a bit tight from coating such that when the barrel nut was "tight", it hadn't tightened down the barrel enough.

Coated the threads with anti-seize and spent an hour tightening and loosening the nut (without the barrel in place) a little bit more each go. After that, I was able to completely tighten the nut down onto the barrel properly.

[bmiller]

With my seekins hand guard that is not the issue.

[RogueNathan]

Purchased a Faxon gunner barrel. I really like the barrel, and how it handles in the gun. I am having a problem with a wandering zero. I am using a seekins triangular handguard. I am also using an older m&p upper. The barrel extension did not feel super tight sliding into the upper. Could shimming the barrel fix my problem? The barrel seems to shoot well, but the groups wander. The picture is a group I shot prone with a block of wood as a brace under the forearm at 200 yards. Problem is it is not where I was aiming! Does anyone have experience with this problem?

Thanks

It sounds like to us that its an issue with the interface of the barrel to receiver to extension. There are a couple things that may be the issue.

What I can comment on is our extensions. We test each one using hardened steel gauges to ensure perpendicularity on the front and rear of the flange. Only barrels that pass are allowed to make it to the field. While this does not absolve us, my reasonable assumption is the issue is outside the barrel.

1. We understand that the upper previously had another barrel installed on it. The receiver and barrels (especially if there was high round rounds) can "mate" to one another through repeated compression (especially on the aluminum side). We would encourage you to look at the receiver and place on a very flat surface. See if it is anything but perfectly straight up and down or if there are light gaps. That would indicate an out of spec upper.

*On a side note, I am inclined to lean towards this as when you increased compression on the barrel, it tightened up.

2. Likewise, the barrel nut can also have the same issue but it is very difficult to test. Considering its material (either steel or titanium), it is unlikely it is an issue if your previously barrel worked well.

Now, if the issue does turn out to be the barrel, I will gladly replace it.

[bmiller]

I measured my upper. It measures 1.01. Barrel extension measures .9975. It is floppy without nut.

[mpom]

Seems the upper is the culprit, not the barrel extension.

Just checked a couple of barrels I have laying around, 2 different reputable manufacturers (like Faxon) and they came in at .9995". No stripped upper at this time, but believe the mil spec for bore where barrel extension sits is 1.0 +.002, -.000

So your upper opening is .008 out of spec.

If it was me, I would try a retaining compound first, then a new upper. Unless you fashion a shim that is symmetrical around the barrel extension, suspect you will introduce harmonics and support issues that may impact maximum accuracy.

A feeler gauge shim would be appropriate for a Ruger 1022, but imho, not for an AR15.

Loctite 620 is suitable for up to .015" gap, so should work. 609, which I mentioned earlier is only good for up to .005"

[bmiller]

I talked to Nathan at Faxon and he has been very helpful and available the whole way. He is thinking the upper as well. I have a new billet upper that I am going to try next.

[mpom]

Nathan is a class act. If I were in the market for a barrel, Faxon would be one of a small # I would look at. Wish they made a 16" with intermediate gas...

[wbear]

A BCM upper will cure that problem, they are all I use now.

The barrel hole is purposely made slightly undersize. The spigot is heated to around 250 or 300 degrees (I use a heat gun) and the barrel will slide in. After cooling there will be a tight fit with no play, no messy compounds and no shims.

I also use Faxon barrels.

[RogueNathan]

Nathan is a class act. If I were in the market for a barrel, Faxon would be one of a small # I would look at. Wish they made a 16" with intermediate gas...

I am flattered beyond measure. I just hope I can keep this up as the company grows!

A BCM upper will cure that problem, they are all I use now.

The barrel hole is purposely made slightly undersize. The spigot is heated to around 250 or 300 degrees (I use a heat gun) and the barrel will slide in. After cooling there will be a tight fit with no play, no messy compounds and no shims.

I also use Faxon barrels.

Great suggestion!

-Nathan

[mikeAZ]

Geeeezz, I may as well get on the Faxon bandwagon?.... 2 on the way

[sschultz]

mpom not sure what harmonics are involved but Jp uses that same shim system with there barrels so it shouldn't affect it to much.

[Vlad]

A BCM upper will cure that problem, they are all I use now.

That has NOT been my experience. I have a couple of BCM uppers and both were oversized on the interior diameter. My guess is you will see random sizing depending who made them for BCM that week. For me the JP ones seem to be more consistent.

[mpom]

A bit of thread drift.

sschultz or co-express, what kind of shimming does JP use to eliminate play between upper and barrel extension?

Thanks in advance.

[wbear]

A BCM upper will cure that problem, they are all I use now.

That has NOT been my experience. I have a couple of BCM uppers and both were oversized on the interior diameter. My guess is you will see random sizing depending who made them for BCM that week. For me the JP ones seem to be more consistent.

I think your experience is with their older uppers. Trust me their latest version fits TIGHT! If heat isn't used the barrel will have to be forced in. I bought my 3 last fall and this winter and installed 2 Faxon and one Voodoo barrel in them. Try a new one you might be surprised.

Here is some info on it from another site:

"The good folks at BCM were kind enough to send me a sample of their new BCM M4 Upper Receiver. "Another boring M4 Upper Receiver you say?" Well, not quite. This one has some advantages over all other ones.

Back story, BCM has been buying upper receivers out in the OEM market place for some time. They have discovered a problem with this. First is the fact that AR parts manufacturers will ship you whatever they feel like (meaning don't care if the part is in spec or not). BCM has had to reject upwards of 50% of the upper receivers they get in because they did not meet their specs and standards! Things like the Picatinny rail slots are off or not square and true or the inside dims are not right and your BCG is too loose or too tight, etc, etc..

Having had enough of this, BCM is having a machine shop make these uppers to their drawings. Lots of QC procedures are in place to ensure everything is correct (pics on this to come). There was one small change made from the TDP on these receivers. They made the Inside Diameter for barrel extension slightly undersized for a tighter fit to the barrel. By doing this, you will improve the accuracy of your AR."