Hello guys, I have been searching around various builds and articles on this forum and others concerning the subject of AR barrel installation. There appears to be a number of methods ranging from a simple dry installation all the way to machining the barrel extension flat. For a competition/multi gun configured rifle, which will likely see a barrel change or reconfiguration a time or two, what method has been most successful? I ask because I have seen a number of people using red or blue loctite inside the barrel extension, which from my experience is a nightmare to remove down the road. I have seen grease applied to in the flange on the barrel to avoid damage when tightening the barrel nut, as well as grease in the barrel extension. My JP barrel fits in my Aero Precision Enhanced upper without effort, which in my opinion is not very desirable. I would prefer there to be slight resistance to avoid play in the connection. Some say the Loctite combats that play, and provides a better connection. A lot of methods, any preferred? Thank you guys in advance

I was taught to lap the reciever to make sure it's square then blue loctite on extension.

Has served well for me with a few builds. I did remove one barrel after using blue loctite and it was a little tough.

I also grease the threads of the barrel nut.

There was a great video on YouTube that covered this. If I remember correctly the man in the video was an armorer/gunsmith for the Army Marksmanship Unit. He tells a story about soldiers cleaning their weapons and noticing that when oil was in their chamber area, and the barrel was flexed or moved, bubbles were present indicating space between the barrel extension and the threaded portion. A quick and relatively easy fix was very thin shims of metal. The smith recommended a slightly oversize barrel extension.

BCM uppers are reputed to be very tight.
The two I have bought certainly are.
I don’t use it myself, but I have heard the competition guys like Loctite.

Here it is. LINK

Once the barrel nut is secured, the barrel shouldn't move in the upper.  That doesn't mean that flexing the barrel won't cause the receiver extension to flex inside the upper though. While it won't be much, in a precision build it may be enough to give the builder some concerns.

If there is enough room between the upper and the barrel extension for it to move noticeably while you're installing it, or if the barrel extension seems to slip in too easily, you can "shim" it with plumbers' teflon tape - it should only take a couple layers of the stuff to make the extension fit snugly in any upper I'd want to use in a build.  In other words, if a couple layers of teflon tape don't make the barrel extension really snug, there's something wrong with either the barrel or the upper.  

The only drawback of using teflon tape is that it will probably burn off after a while - standard 3 mil tape is rated only up to 500ºF.  But there's something else that will also work: aluminum foil tape.  This kind of tape is 3.6 mils thick, and while the adhesive is only rated to 600ºF, the adhesive won't matter once the barrel is installed.

I used lock tight when I installed my JP barrel.

Thanks guys, I actually just test fit my JP barrel in another receiver, and it slips in with no effort as well. Im guessing due to the uncoated stainless surface, these barrels provide no interference fit with the receiver. I am very hesitant to use loctite on the barrel

Just my 3 cents.


If your going to lap your upper remember your only looking to remove any of the high spots


be very careful if you lap DO NOT over lap it you can do more harm then good if you do.





as for Loctite IMO i don't like to use it on the barrel or barrel nut but im not building HIGH precision bench rest or ultra reliable tactical ar right now ether

if i was i probably would just for the little extra peace of mind.



if you think you'll never take the barrel or nut off again go for it, but its been my experience that for me i end up taking

thinks apart and putting them back together again more then i care to admit lol so i only loctite some things.



but again for me it mostly depended on what kind of build im doing ya know from a Tactical rock crusher to a ultra light race gun.

different methods for different needs



in short IMO maybe you do and maybe not just depends on what you want your finished build to be.



Lock load live free

I was about to suggest the aluminum tape cut just enough to wrap once around the extension. Heat the upper receiver and cool down the barrel. Should slide right in without messing up the tape. Once the temperature equalizes, then it should be a really tight fit.

Lap.. period.  When you do though take it until you get obvious touching around 90% of the circumference of the upper face.  Using this standard prevents you from going too far, and is plenty good to achieve the desired effect.

My BCM upper was quite tight inside.  Took effort to get the lapping tool into it so I lapped the inside of the upper ever so slightly as well.  Made for the slickest action I have.

When tightening the barrel nut use the minimal torque necessary.  Excessive torque can apply torque to the upper itself that can change as things heat or cool.  By using the minimal necessary torque (close to 30ft/lbs as possible) this minimizes the chances of torque-induced inconsistency.

When folks talk about the use of blue Loctite they are talking about on the barrel extension where it is inside the upper, NOT on the barrel nut.  This is to create the perfect fit between the two  Looked like some may have been misunderstanding where the Loctite was to go, soooooo.......

Thanks for the link... very informative.

My sample size on BCM uppers is four.  They were definitely on the tight side of spec.


FWIW, OP, why not buy a new barrel extension (large size of spec) and replace it?

Instead of aluminum tape, what about something like this? (The gunsmith in the video mentioned shim stock)

http://www.amazon.com/Aluminum-SAE-AMS-DTL-22499-Length-Laminate-Layers/dp/B005TMUVH6/ref=sr_1_4?ie=UTF8&qid=1447190798&sr=8-4&keywords=aluminum+shim+stock

Extensions are essentially a permanent part of the barrel.

Use shim stock on your extension and put green loctite on it and you'll be fine

Any advantage to aluminum shim stock vs steel?

I've never had to use it as the 3 uppers I built like this were tight enough to only use the green loctite.

But if i needed to use shims I would use aluminum

We are talking about shimming a VERY small space.  Aluminum shims (or tape) are available in very fine thicknesses, while steel shims are not.  Basically, what we're talking about is a situation where you might need something the thickness of foil to fill the gap between the barrel extension and the upper because the barrel extension isn't super tight in the upper.

Green Loctite 290 is a "wicking" threadlocker intended for application to preassembled fasteners.  It is easily removed with a heat of 482ºF.  Green Loctite SLEEVE RETAINER 640 is a "high temperature" adhesive for bearing sleeves and similar applications - but it's only good to 400ºF and breaks down for disassembly at 250ºC (482)F).  Neither product is heat-resistant enough to withstand even a modest firing schedule without breakdown.  This is why I recommend skipping it altogether and just going with foil/tape.  Once you assemble the barrel to the upper, unless you don't believe in torquing the barrel nut, the barrel won't move on its own.

There are a number of threads on the subject of shimming the barrel with liquids and other materials right here is this subforum.

Here are a couple of these but there are several others if you want to Google the subject.

Rocksetting a AR barrel
Anyone shim your barrel extension

I helped a buddy install two barrels this Veterans Day on his CMP Service Rifles. One was a 1/7 Shilen barrel from WOA and the other was one of the house brand (Wilson blanks) 1/7 WOA.  

We used .001 stainless steel shim stock I got from Enco. We cut a strip the width of the barrel extension to the locating pin and long enough to go all the way around without overlap.

The WOA Wilson took a length all the way around and the fit in the receiver was very tight. It took four hands to hold the shim stock in place and my buddy to slowly work the upper back and forth by rotating it and providing a little pressure to move it forward up to the torque shoulder on the extension. It took about 10 minutes. If you just try and push it on the shim stock will wrinkle and move. The last .100 we had to use a 2X4 to bang the upper all the way down. No wrinkles on the shim though.

The Shilen didn't need a length of shim stock nearly as long. Basically a 1/3 diameter pad on the top of the extension. Still needed four hands to hold the shim tight on the extension and working the upper onto the barrel.

Once everything was screwed together we put a little green (wicking) loctite into the small gap between the extension and the receiver on the inside of the receiver.

This is the third barrel I've done like this. I have two other CMP Service Rifles I'm going to disassemble and re-install with this technique.

My target rifles are good shooting guns. I made the P100 this year at Perry and shot some cleans at 600 yards with my 6mm RAT. I'm convinced there is a correlation between extension fit and superb shooting guns.

I don't think I'd do it on a go-to-war gun, but there are other ways to improve extension to receiver fit. BAT Machine makes some oversized extension that can be used if you use a smith to put together a barrel.  

B

Re-installed a barrel on a gun with a FF quad rail and took some pictures. It took about 10 minutes to work the upper onto the extension with the shimstock in place.

Thanks for the pics. So it looks like you didn't have grease or locktite between barrel extension and shim or the shim and receiver extension? Totally dry installation?

The last three I have done have been completely dry. The SS shimstock seems to have a pretty good affinity to staying in place as I'm working the receiver down around the extension. Four hands are better than two, but I managed to get this one on myself. Basically I have to pull it tight around the extension and then gingerly work it into the receiver by rotating back and forth.  

Once I get everything screwed together I use a little green wicking locktite from the other end and let it wick into the small gap between extension and upper receiver.

The first one I did I used some blue locktite to try and stick the shim onto the extension. It worked but it buckled a couple of times and was messy. I think I trashed the first shim. Completely dry assembly seems to be the smoothest.

B

Why not apply a release agent and use JB weld?  600 F operating temperature.

Just get a different upper. They are cheap in the scheme of it all. BCM and Vltor, to name a couple, machine to the tight side to ensure the barrels fit properly.

Just a thought, after dry fitting have you ever tried putting the barrel in the freezer and the upper in a warm oven so that the barrel shrinks and the upper expands by a few thousandths so it's easier to get the barrel in without wrinkles in the shim?

I have to wonder what you're doing to the barrel and upper when you have to pound the barrel into the upper...

With that said, I also don't think you gain any advantage in accuracy by shimming so much that you have to force the barrel in either.  The 3M aluminum foil tape I mentioned earlier is 3.6 mils thick - that's 3.6 TEN thousandths of an inch thick, which makes it about a third as thick as the 0.001" stainless shim stock bpm990d used.  3M's stainless tape is 3.8 mils thick, so essentially the same as the aluminum tape.  Both tapes will stay put when you apply them, which also helps prevent getting fingers pinched while assembling the upper.

The point of shimming the barrel is to remove slop in the way the barrel fits in the upper.  If you are trying to build a precision upper, I can't see any good coming from having to pound it together...

It's not some wilting school girl that bruises easily. The barrel is designed to take 55,000 PSI routinely and the upper is 7075. A couple of whacks on the ass end of the upper receiver with a 2X4 isn't going to do anything. What exactly do you think I'm going to damage?

Have you read Joe Carlos' article The Relationship of Barrel Extension Diameter to Accuracy in the AR15 in American Gunsmith? Joe found a 13% improvement in an entire fleet of target rifles by improving the fit. This is the difference between a low X count clean, and a high X count clean at 600 yards with iron sights.

Wonder all you want, it has no effect on the results of any match or the precision of a target rifle. BTW, I'll have to check out that aluminum tape, but I have enough SS shim stock to do a hundred rifles and any new barrels will have oversized BAT receiver extensions.  

B

Good idea. I'll have to give it a try. I've got two or three others I'm going to take apart and re-do.

B

I am NOT suggesting that a firm fit between the barrel and the upper isn't a good thing.  However, I worry about crowns, about the impact bending the upper, and I think that any technique that requires that much effort is "overdoing it."

I do not doubt that improving the fit of a barrel to an upper is useful or worthwhile.  I just don't think that using an oversized shim - which requires the use of drastic means to install the barrel - is a good choice.  Particularly when other shim material (like both types of metal tape I mentioned) would work well to give a solid fit between barrel and upper.  What was the difference in diameter between your upper and the extension of this barrel?  Adding 0.002" worth of shim appears to have made for a fit that goes beyond "interference".

Your reaction to my post seems extreme.  What is it you think I was trying to say?  If you reread the whole thread, you'll see that I have given suggestions for simple and easy to do methods for improving barrel-to-upper fit.  I tend to go with a more cautious approach with this sort of thing, particularly if my purpose is to get the best precision out of my build.

It took all of about 10 minutes on each of the last three rifles I did. It's not a lot of extra work. I didn't pound on the muzzle to get the barrel into the receiver. Honestly, did you really think that and why would you bring that up? I used my hands to get it on, but it needed some rotation back and forth to get it to move forward. Once you get up into the indexing pin, you don't have a way to wiggle it back and forth. I whacked on the back end of the receiver to seat the front end up against the shoulder of the extension.  There is a reason smiths have about a dozen different hammers and mallets, gunsmiths have been whacking on things since the beginning of time.

Is this technique a little fiddly? Yes, and if you cut a length of shim that it too long, it may be more of a challenge and require more effort. You can always trim it shorter and have an easier go of it. I'm sure your aluminum tape works well too. I'm using the shim material because that is what Joe Carlos used and I purchased right after I read the article.

B

let me know how it goes. I have to do a precision build in January and was going to tack the shim down with Green locktite then use the hot/cold method to fit it in.  I am hopping to get a slight interference fit that way but if it's easy enough to slide in this way I can scrap the locktite and not worry about it wrinkling with just my 1 pair of hands

I didn't think you hammered on your barrel or your upper.  But having been around here for a while, I thought someone would do that, and I am sure someone would read what you first wrote and think exactly that.  I was posting about a "worst case" situation, where Bubba and Slim decided to do "whatever necessary" to jam a barrel into an upper.  They'd probably use duct tape instead of stainless shim stock, and you can see where that could go downhill really fast.  YouTube material...

Did you measure the extension and inside of the upper?  If adding 0.001" shim (0.002" in diameter) made for that tight an interference fit, your barrel was probably just barely "not tight" to begin with.

Have you wrung out that upper yet?  What kind of accuracy did you get?

Also interested in before and after accuracy comparison, since it was mentioned that you were going to add shims to previously assembled rifles. The armorer on YouTube from the link posted earlier in this thread mentioned 1/3 reduction in group size, iirc.

No I didn't measure the upper or the extension. It was a slip fit like most barrels. It came off one my backup target rifle and probably has less than 1000 rounds through it.

I have not shot it. I'll post some groups or scores when I do.

B