Accurizing AR barrel install ? Is there such a thing ??

M

Metal god

New member
I'm about to put together my upper for my high power competition rifle . Is there anything I need to do special like bed the barrel extension receiver area or do I just put it together like a regular AR ?

Also would like to know if there are any measurements I should do before putting the upper together when it's easier to get at everything ? Like getting a mold of the chamber , head spacing or fit of the bolt to the barrel extension ?

The barrel is an ER shaw 20" HBAR 233 Wylde 1-8 5r . If that matters
 
Some people say that an AR will never perform its best with a barrel nut torque over 40 lb-ft. -So, you should lap the receiver until you are able to achieve that.

Others say that RTV "bedding" is necessary for any barrel extension that doesn't require heating the upper for a press-fit.

And, you can go on ...forever... with everyone's tips and tricks. ...Most of which have as many exceptions as proofs.


But, if you're going to check headspace, it's definitely advisable to do so before installing the barrel. It's much easier to do, and will save you from wasting time with assembly and disassembly, if the barrel checks bad.
(Something I'm kicking myself for on a build I finished Wednesday, and I'm not too happy about.... :rolleyes:)
 
I'd make sure the barrel's groove diameter is a few ten-thousandths inch smaller than the bullet diameters you'll use.
 
What's RTV bedding ?


A few ten thousandths ??? That sounds kinda vague . Iwas going to start with 77gr smk and also try Bergers 75gr match bullets . Any others I should try ?
 
Jacketed 22 caliber bullets need to be about .0002" to .0003" bigger in diameter than the barrel's groove diameter to shoot the most accurate. Even as much as .0010" larger and they'll still work great.

Anything undersize has never proved best for accuracy. If you're willing to accept no better than about 1/2 MOA at 100 yards, then so be it. I wouldn't shoot Sierra's HPMK bullets in an oversize bore. You'll get the same accuracy with their hunting bullets.
 
Sorry Bart I misunderstood or misread your measurement . For some reason in my head I calculated a few ten thousandths to mean 30 thousandths :eek: oops :o Now it's not so vague any more .

If you're willing to accept no better than about 1/2 MOA at 100 yards, then so be it.
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LOL I can barely shoot 1/2 moa on a bench with a 14x scope . 1/2 moa off hand at 100yds just seems like a pipe dream right now . Maybe some day but right now that would be a tall order for me . To be fare to to me , I've shot maybe 300rds with open sights at 100yds in the last 2 years and about 150 of those were the last two time I went shooting . Some real concentrated practice will help I'm sure and that's what I plan to do onces I get this rifle up and running .

I see in your second post you mention 22 cal bullets for those measurements . What about 30 cal (308) ? I ask be cause my Ruger American seems to have a tighter bore then my Savage FCP-K . The reason I think that is when I pull a bore snake through the ruger It take quite a tug to get it through . How ever when pulling that same bore snake through my savage it glides through like butter with very little effort .
 
What's RTV bedding ?
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Coating the barrel extension in RTV, to fill in the gaps. Alternatively, some people use Loc-Tite (Red, in particular).




Back on the topic of lapping....
I lapped the A3 upper receiver for a 5.56x45mm mid-length 16" build (with a ridiculously low quality barrel), using the Brownell's lapping tool and 600 grit compound.
It was doing no better than 10" or larger groups at 100 yards, as-assembled, with quality match-grade ammo.
After initial lapping and a barrel nut torque of 80 lb-ft, the groups shrank to 6-7" at 100 yards.
Further lapping, to get to a barrel nut torque of 35 lb-ft, cut group size to 3-4" at 100 yards.
I'd like to imagine that that would translate to a 1 MoA rifle shrinking groups to 0.3-0.4 MoA. ;)

So, I'm a believer in lapping, if you don't already have a high quality upper receiver and barrel extension.
And, it may lend some credence to the low torque theories.
But, as shown by that terrible mid-length barrel I used.... you can't polish a ****. ;)
It might help a decent barrel, but it won't turn a PTAC barrel into a Noveske or FN CHF Select.
 
So what do you think is happening with to much torque on the barrel nut ? It's an interesting thought . On the last barrel I installed for a mid length 16" , also installed a YHM rail ( Tod Jarrett ) and I had to put quite a bit of torque to get the gas tube lined up . It's no P-Tac so it shoots fine but it is something I'll keep in mind .

As for the RTV . Will I be able to get the barrel off latter or is it permanent ? I aslo heard to use sleeve retaining compound http://www.permatex.com/products-2/...matex-high-temperature-sleeve-retainer-detail is that the same as RTV

Any ideas on what to use to get a cast/mold of my chamber and bore . Or what's the best or easiest way to to that
 
Metal god, 30 caliber barrels are same as 22 or any other caliber. They need groove diameters a few ten-thousandths smaller than bullets. The British arsenal 7.62 NATO bullets they have to shoot in their fullbore long range matches are often .3070" diameter so they use barrels with groove at .3065". Great accuracy with a good lot of ammo.

Contrary to that, Winchester 70 factory match rifle 30 caliber barrels were about .3083" in the groove, so arsenal. M72 or M118 match bullets at .3086" diameter or WCC match 197 HPBT or Lapua D46 185's at about .3090" shot most accurate in them. Sierra's HPMK's at .3082" were marginal at best.

John Kreiger knows all about this; it's the reason his 30 caliber match barrels have .3075" groove diameters. He'll make 'em down to .3065" if needed.
 
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I'm not advocating RTV. I just brought it up as an example of what some people think works.

If you want to try it, I would suggest using RTV over the sleeve retainer compound. That sleeve retainer is pretty much a permanent installation. It is TWELVE times stronger than red Loctite (#271), which is, itself, generally considered a permanent installation (barring the application of 500+ degrees F to the upper receiver).
RTV is just a silicone sealant, and will release with much less effort than any threadlocker or similar product.



Cerrosafe is your best bet for a chamber and throat cast. You'll want to be very careful not to pour the cast in any way that will interfere with the locking lugs and recess of the barrel extension, though. If that happens, the cast will have to be melted out by heating the barrel.

It also shouldn't be considered a permanent cast. You cast the cavity you want to measure, measure it within the specified time frame, and rely on your recorded measurements in the future. The cast itself will be oversized and warp over time, so it isn't a suitable long-term reference.
 
So it's ready to go together but I am going to make a mold of the chamber and throat first .

Couple things real quick . Is the bore just ahead of the throat good enough to get my grove measurement or do I cast the muzzle ? Muzzle sounds like a NO NO but thought I'd ask .

The other is , I plan to use some sort of gasket /sleeve compound on the barrel extension receiver area . I'm thinking of first putting the rifle together with out it . check function with a hundred rounds or so. Then take it apart and secure that connection better . My thought was if there turns out to be a problem and I need to take it apart it won't be so hard to do so .

I checked head space and that's good . Only had a field gage but that should mean I'm safe to shoot ?? Yes ??? GO gage is on back order . I made some dummy rounds with the same spec as my plinking rounds . They manually cycle just fine .

Anything else I should do before I put this thing together let me know

Thanks Metal
 
It's always useful to know what the muzzle measures, but casting the chamber, throat, and a small portion of the rifled barrel just ahead of the throat should give you a good idea of what you're working with. (I'd say no more than 1/2" would be necessary.)


So the field gauge didn't fit, but your dummies are cycling?
 
So the field gauge didn't fit, but your dummies are cycling?
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Correct , with a heavy hand tight barrel nut . The upper is not really together so I'm doing what I can with what I have . The bolt will not close on the field gage . I'm just pushing the BCG forward by hand and pulling it out but I did try to force the field gage , it was having none of that . The bolt closes on the dummy rounds and when I pull the BCG back the round ejects like it should through the ejection port .
 
To properly check headspace, you need to fully disassemble the bolt; and it's easiest to check with the barrel removed from the upper.

Remove the bolt from the carrier, and remove the extractor and ejector from the bolt. Clean the bolt face, clean the chamber, and clean the gauge. Then, use the gauge with the bolt hand-held.


Make sure everything is the same, stable temperature, as well.

Today, I found out that that's the problem with the barrel I didn't check before installing, which had short headspace: The company rushed my barrel through production, and checked headspace immediately after reaming the chamber, while the barrel was still warm. Headspace was good with the barrel warm, but the chamber 'shrank' when the barrel cooled. So, now it's short. :rolleyes:
 
The thing that holds me back with my AR15s is the chamber.

If I put an old 22 rimfire barrel on an old mauser stub with loctite [cut no threads assemble like tinker toys], but my 223 reamer, I can build a $50 + scope sub moa rifle.

But my AR15s that cost over $1k, cannot shoot that well [right at 1.1 moa again and again and I have really tried]. It is just too far to the lands in that NATO chamber.
 
But my AR15s that cost over $1k, cannot shoot that well [right at 1.1 moa again and again and I have really tried]. It is just too far to the lands in that NATO chamber.
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So, try a different barrel, with a different chamber. ;)
 
here is a cast of my 223 wylde 5r chamber , throat and bore . The jump to the lands does seem quite a bit away .

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This is for my national match service rifle so I'll be shooting irons with it . I'll put a scope on it some day but it may be a while before I know how accurate the barrel/rifle really is .
 
There are a bunch of different ways to install an AR-15 barrel with a variety of results. The rate of fire has an effect as well.

In any case, the best bet is a slightly oversized extenstion, or undersized upper. That will allow some interference and let you thermofit the barrel, which is the best plan of action.

RTV and Locktite are not a good idea. RTV has no strength, Red Locktite only works on non-passive surfaces. Lapping removes the anodizing, making the surface softer, and you also set up a very active corrosion cell between the exposed aluminum and the stainless extension.

Anti-seize on the threads and a minimum of 3 torques to about the level you want before the final torque. You can use shims or turn the mating ring of the extension to get the torque level you want. Anything under 40, to me is asking for trouble down the road.

I am currently shooting an AR-15 that was thermal fit, torqued 4 times with a final of 105 ft-lbs, and it only shoots 1/2 MOA, but no thermal drift at all. I can shoot a 5 shot group, dump a 30 round mag through it and then shoot another 5 shot group that will print the same location and group size. It has just about 10K on a match barrel and is still holding.
 
I agree that some 'thermal assistance' results in the best fit, but that's rarely encountered outside of high end uppers specifically milled with tight tolerances, or with barrel extensions specifically milled to be oversized.

Honestly, I was a bit disappointed when my 6x45mm barrel was a snug fit, by hand, in the Mega SBU upper I selected for the build, rather than requiring some heat. :(


Lapping removes the anodizing, making the surface softer, and you also set up a very active corrosion cell between the exposed aluminum and the stainless extension.
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Whether or not the lapping process breaks through the anodizing depends on how thick the anodizing was to begin with, and how far you need to go with lapping.

The MIL standard for type II and type III hardcoat anodizing calls for 0.002" (+/-20%) standard, but allows for any thickness to be specified. As such, manufacturers can anodize to any thickness they want, and still claim that it is MIL-A-8625, type whatever, class whatever, without lying. I've seen upper receivers with anodizing as thick as 0.008" that were sold as 'hard anodized per MIL-A-8625' in one place and just 'hardcoat anodized' in another.

Given that you only have to remove 0.00277" to turn an AR barrel nut one full notch/hole, and that most people lap only to true the face of the boss or to get a partial turn of the nut, lapping generally removes less than 0.002" - often less than 0.001".

If you need to go farther than that to true up the face, then go to a different upper. And if you need to lap farther than that for the desired torque value, then you can also use shims - or even should have used shims.


And... you need moisture in contact with both metals, for galvanic corrosion to be an issue. Unless you make a habit of soaking your AR on a regular basis, AND lapped all the way through the anodizing, it isn't something to waste time worrying about.
 
RTV has no strength
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Real question not arguing . What do you mean . I was thinking it did not need strength per-say because the barrel nut is whats providing the strength . What ever compound if any I use would be used more as a shim then anything else . Are you saying it's to soft therefore it could still move around ?? or to brittle and would break apart after a while .

How do you shim the connection and with what ?? I have some feeler gages as small as .0015 but I don't think there stainless .
 
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