Curious, how loose is too loose? I'm planning an SPRish upper build, and the barrel-to-upper fit is something I could charitably describe as loose. Almost no effort required to insert the barrel, and the barrel would fall out if I turned it barrel-down.

Is this something that the barrel nut will solve? I note in my research that people sometimes advise putting loc-tite on the receiver extension to help maintain the consistency.

Should I look for another upper that may have a better/tighter fit?

Thanks, any help appreciated. I am still in the process of acquiring parts and want to get good stuff to build this with. GIGO and all that.

I have a Triton barrel one like that.

It seems like my tight ones, accuracy wise.

coat it with Loctite  sleeve retainer   number 640 before putting in receiver.


this will fix the loose fit after it cures and will increase accuracy.

Righteous - thanks guys. Never built an upper, and want to build the best I can.

Out of curiosity, how good would that be?

Is Amazon's price of $176(!) indicative of how much this stuff runs??? I could buy a couple new uppers for that.

Your probably looking at the huge bottle and not the little tube.

Here you go.
http://www.amazon.com/dp/B0002KKTIG/ref=twister_B00E618ALU?_encoding=UTF8&psc=1

I've been contemplating doing the loctite thing, I just got a barrel that is pretty darn loose and wobbly.

I think the barrel extension is slightly undersized, it's really loose in the 2 flat top receivers I tried it in and still a little wobbly in a M16 upper that any other barrel needs a little coaxing to get in.  

I thought about buying a BCM upper receiver because they're supposed to have a tighter fit, but I'm not sure it would be enough and I don't really want to buy another upper when I already have the one I planned on using.

Is 640 for sure the stuff to use?  I read some some recommendations for 609 or 690 and it seems like a bunch of #s in the 600s are similar.

this


the 640 is the way to go it has 400 deg heat resistance and the thickness is just right.  they make a few slightly thicker blends but they are "only" 300 deg formulas.

Thank you!

I would try the BCM upper instead of messy loctite unless you are certain of never taking it apart again or are going to sell it. IMO a tight metal to metal fit is superior to a substance filling in the gaps. I'm not saying the loctite won't work, it's just not for me.

I tried 4 barrels (2 Faxons a Voodoo and a Bushmaster) in a Spikes, Bushmaster and Anderson upper. All dropped in with what I consider a "loose" fit.
I bought 2 BCM "tight" fit uppers. None of the 4 barrels would drop in either upper and all required the upper spigot to be heated to 300 degrees with a heat gun for insertion.
Removal required reheating due to the nice interference fit.

Unless your barrel extension is way out of spec I think the BCM upper will give you a tight fit. It sure did for me.

 FYI, sleeve retainer material will permanently fix your barrel to your receiver.  That's the stuff they use to put iron cylinder liners in aluminum engine blocks.  A better approach, IMO, is to square the front edge of your receiver so the barrel nut has a near-perfect flat surface to seat against.  If you really want to coat the exterior of the barrel extension for added firmness, by all means use blue Loctite, not the permanent stuff. - CW

http://www.midwayusa.com/product/821295/wheeler-engineering-delta-series-upper-receiver-lapping-tool-ar-15?cm_vc=ProductFinding

Seems like I've read from folks that have done it, that it's not permanent, and you can just use a wooden dowel to knock out the barrel from the receiver?  

Facing the receiver isn't going to change the dimensions of the barrel extension that's just floating in the receiver,  I don't know for certain if it's bad for accuracy, but doesn't seem like it would be good.

I haven't encountered a receiver that was out of true that would need to be trued.  Seems like a separate issue.

This...or plan on never removing the barrel.

People who have done "non-permanent" Loctite installs have used Loctite threadlocker, NOT the sleeve retainer mentioned in the early post.  The "I'll just use red Loctite and make it snug" folks have the wrong idea, at least in my opinion.  

The tightness of the barrel's fit in the upper doesn't have anything to do with accuracy.  It only has anything to do with sight alignment with the barrel.

Instead of using messy stuff like Loctite, you can get the barrel to fit well in the upper with plumber's teflon tape. This will help keep the barrel centered in the upper while you're assembling it.  Wrap two layers of teflon tape around the chamber end of the barrel and you'll change the fit substantially.  This tape will melt with firing, but once you've properly torqued the barrel nut, it won't make any difference - the barrel will be solidly mounted in the upper and it won't move.

No wonder plumbers make the big bucks.

A barrel extension that fits "loosely" in the upper is a non issue once the barrel nut is tightened.  The clamping force of a barrel nut torqued to 80 lb-ft with lubricated threads is approximately 24,000 pounds.  Your barrel is not going to move on the upper receiver with a properly torqued barrel nut.  In my expert opinion, the use of Loctite is bullshit and only complicates later removal of the barrel.

I'm going to disagree with those that say a tight fit between barrel extension and receiver does nothing.

I was just at Camp Perry and White Oak Armament is now selling a newly designed, oversized barrel extension. It also requires a longer threaded tenon than a regular barrel extension and I really like that a lot but I don't believe they are advertising them on their website yet. You can call them to find out more, but why are they doing this? Why does BAT Machine make oversized barrel extensions?

Watch this video for Joe Carlos where he talks about the fit of the barrel extension to the upper. https://youtu.be/pJL-IscH_jo  If you are really interested, get a copy of some of Joe's articles from American Gunsmith magazine where he discusses this in depth. The article is The Relationship Of Barrel Extension Diameter To Accuracy In The AR-15, Part I & II.

For those that don't know, Joe was the armorer for the USAR Service Rifle team and once held the Interservice 1000 yard Service Rifle record (albeit with an M-14). He is a US Army Distinguished Rifleman and a member of the President's Hundred.

Bottom line is he has tested it in numerous rifles with a return-to-battery machine rest and it makes a significant improvement in group size (about an 18% reduction).

B

A competition barrel, in a rifle with competition sights, in the hands of a high-level competitor who's shooting precision-crafter ammunition will have a lot of advantages.  Having the barrel extension oversized, so that it's hard to insert in the upper, is not a useful thing for almost anyone else.  It's like saying that the barrel extension, all by itself, makes the combination of rifle, sights, ammunition and shooter all that much better.  It might help the competitor a bit, but it's not going to help Joe Schmo shoot his TulAmmo any closer to MOA.

As long as the barrel is kept centered in the upper while the barrel nut is properly torqued, there's no practical impact on the inherent accuracy of the barrel OR its alignment with the sights.  Of course, if I were paying big bucks for a barrel that's supposed to be extremely accurate all by itself, I would want the barrel extension to be at least within specs - which means that it should be snug in the upper.  

Frankly, I've yet to have an issue with a too-loose barrel extension.  Most of the barrels I've installed have needed at least a little grease to allow me to insert the barrel in the upper, with the remainder needing not only some grease but being perfectly aligned from the beginning.  To me, it's really a matter of whether or not a barrel that's "loose" is actually loose, or if it's just not "too tight" to go in without a light touch and some grease.

So - professional opinion here - if I'm building a Mk12ish rifle, with an accurate 18" barrel, and want to get the best groups I can possibly get, I should shim or figure out a way to make the barrel fit tighter in the upper?

For some background: I'm just a schmoe, but I love accurate rifles. I can (off a bench) shoot an honest sub-MOA, my best group is 1/3MOA. Will a looser fit barrel not get me what I want out of this upper?

Is this a new barrel?  Is it going in a new upper receiver? To get the most out of the combination of barrel and upper, I'd go with a quality upper from a known-reliable name brand.  If the upper is new and you're not happy with the fit, I'd return it and get another one that fits better.

The only shims I'm familiar with are for adjusting how far around the barrel nut goes, not for the fit of barrel into upper.  The teflon tape trick is an effective and inexpensive way to get a barrel more snug in an upper.

From the way you describe the way your barrel fits in this upper, I'll bet that just a single wrap of teflon tape makes that "loose" barrel hard to insert.  If that's the case, that's what I'd go with.  Once the barrel is properly installed, it will NOT move, so your groups won't change just because there was a tiny bit of clearance between your barrel and the upper.  Just make sure you torque it properly.

Roger that, and thank you. Yes, both parts are brand new.

Most people don't do any type of fitting. ARs shoot amazingly well with barrels that are off the shelf. I would suspect the rifles that shoot a little better than most have a tight fitting receiver extension either by design or by luck.

If you get a regular extension and it's tight in your upper then you are all set. If it's loose then you want to either use another upper, or shim it with some .001 stainless steel shim stock. Those two articles from American Gunsmith outline it exactly. If you have access to a library (typically academic institutions or larger public libraries) that subscribes to EBSCO Information Service you can get the .pdf of the article. EBSCO provides magazine/journal subscriptions to academic institutions.  

If you get your barrel from White Oak and ask them for their newer extension, I believe it's .001 fatter than a regular extension. I don't think there is an upcharge for the newer design, but if it is, I suspect it's pretty modest.

B

EDIT: I re-read your OP and I see you already have a barrel. I'd just use some shim stock and a little loctite to keep the shim in place while you fit it.

Much depends on your accuracy standards too. Removal of a barrel with sleeve retainer needn't be complicated, loctite hates heat, and loses it's strength rather quickly when heated. A quick touch with a torch should make removal easier than you might think. You can then soak the front of the receiver and the barrel extension in acetone (after cooling) to allow removal of the residue.

A loose barrel will affect accuracy to a degree, depending on how loose it is, and how steady you are holding it. I would expect it to be less of a factor than loose receiver fit, and we've all seen sloppy receiver matchup.