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Thanks for the inputs and info guys. I will be squaring the face. Vltor makes a top notch upper but I think a little difference in the Extension OD and the Upper ID plus a coat of Cerakote is what I'm seeing. But do believe the face isn't quite true.
So my concern is that when the barrel nut is tightened down the barrel will not be square with the center axis of the upper, carrier, etc. Then with the play between the Extension and Upper ID would be a concern with barrel whip. If the barrel is not square with the upper axis then a barrel whipping around would put more stress on one side of the aluminum barrel nut, upper and extension flange.
No I don't know if repeat accuracy will be affected, but I definitely see how a different impact point would be affected with the first round.
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KTC, with that heavy barrel, you might reconsider and get a Billet upper. JMO. The CMT are a good price, tight fit and rigid. So no worries.
I thought Vltor had Billet uppers but they are not and with the material removed for the modular forward assist option, less than mil-spec support.
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I've contemplated it Tedward. If I go with another upper I would really like a side charger, which brings other cost associated with it. So I don't know.
The 7075 Forging actually has better properties than 7075 billet and the Vltor is nice at beefy at the front and down the side versus a typical mil spec. So I'm thinking it should be OK, but yes I have been thinking about it, but more for a side charger reason and appearance.
You know what happens if I get another upper. I'll have to get another lower to match, then I'll have to get those Cerakoted as well. It all just snowballs, right? Haha!
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customcutter Wrote:KTC,
I checked my CMT billet upper and it is square. However, I had a slip fit, but no "wobble". The barrel extension IIRC was .9965. I decided to put mine together without loctite for now. I wish it had been a tighter fit, but I may do a before and after comparison if accuracy is suffering.
The main reason for doing it is to make sure that the face is square to the bore. Like others have said, it may affect accuracy and also help address the issues with broken bolts. Once I get some "used" brass, I plan on lapping my bolt also to make sure it is square, and has even contact.
Hopefully a CNC machine would have tighter tolerances, however machines tolerances deteriorate and wear, and if the machine has been "crashed" (hit something it shouldn't have been cutting or bumped into something) then the indexing on those 4 or 5 axis's could be off. I don't know how often the heads are indexed, but I'm pretty sure it's not every day at the start of the shift.
If you decide to loctite, as was already said, no open flames (to hot for the heat treat on Al), use a heat gun, heat evenly and penetrating oil. Loctite the barrel extension, not the threads.
CC - as far as acquiring "used brass" to make your bolt lapping tool goes, just size and cut a 7.62x39. you are going to cut it, regardless. That way, you don't sacrifice any Grendel brass.
customcutter
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NF,
First I've got to get some sizing dies. I think I found a few pieces of 7.62X39 in a bucket of brass I picked up at a local range, before they started selling it to one dealer.
Tedward Wrote:KTC, with that heavy barrel, you might reconsider and get a Billet upper. JMO. The CMT are a good price, tight fit and rigid. So no worries.
I thought Vltor had Billet uppers but they are not and with the material removed for the modular forward assist option, less than mil-spec support.
I've only done 3 of the CMT uppers so far. My Shilen was a tight fit. The JP and the Stoner were more like what I'm used to with AR barrels and forged uppers. I'm doing another barrel install for a friend tomorrow. I THINK it's another CMT. Lots of them floating around in these parts right now.
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I'm going check the clearance btwn upper ID and extension OD.
I wonder what the maching toleance is on the extension.
explorecaves
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Keep The Change Wrote:I'm going check the clearance btwn upper ID and extension OD.
I wonder what the maching toleance is on the extension.
I found the machining drawings on google a while back when someone else asked… Let me see what I can find again
Edit: Upper drawing http://www.firearmfiles.com/files/AR15/a...lattop.pdf
Barrel extension drawing: http://www.ar15.com/mobile/topic.html?b=...8&t=633930
customcutter
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So it looks like 1.000 +/-.002 for the upper tolerances, and .9987 Dia -.0004 minimal tolerance would be .9983" on the extension if I'm reading the "fuzzy" print correctly, which is .0003 larger than min spec on the upper???
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Which might explain why some uppers need to be heated and extensions cooled prior to installation.
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Yes, forgot about that. I've seen folks drive them in with rubber mallets, but forgot about reading where they threw them in the freezer.
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customcutter Wrote:So it looks like 1.000 +/-.002 for the upper tolerances, and .9987 Dia -.0004 minimal tolerance would be .9983" on the extension if I'm reading the "fuzzy" print correctly, which is .0003 larger than min spec on the upper???
Thanks for the info CC. I'll check these dimensions to see where I'm at.
customcutter Wrote:So it looks like 1.000 +/-.002 for the upper tolerances, and .9987 Dia -.0004 minimal tolerance would be .9983" on the extension if I'm reading the "fuzzy" print correctly, which is .0003 larger than min spec on the upper???
Seems that if specs will allow the extension to be bigger than the hole in the upper, that manufacturers would write their own specs to make sure it doesn't happen. The last thing anybody wants is a bunch of returns from the field. Looks to me like a real good place to squirt some Loctite on precision builds 
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MikeGideon Wrote:Seems that if specs will allow the extension to be bigger than the hole in the upper, that manufacturers would write their own specs to make sure it doesn't happen. The last thing anybody wants is a bunch of returns from the field. Looks to me like a real good place to squirt some Loctite on precision builds
.0003" is well within the thermal expansion properties of the materials, as pointed out by Nugginfutz. I wish my barrel was a press fit or even one that I had to do the thermal expansion on to get it in. I think it is much more important on the 24" bull barrels than on my 18" Liberty barrel.
customcutter Wrote:.0003" is well within the thermal expansion properties of the materials, as pointed out by Nugginfutz. I wish my barrel was a press fit or even one that I had to do the thermal expansion on to get it in. I think it is much more important on the 24" bull barrels than on my 18" Liberty barrel.
I'm with you there. I'm thinking bedding is the next best thing.
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MikeGideon Wrote:I'm with you there. I'm thinking bedding is the next best thing. I'm afraid I will need to bed as well.
Just from moving the barrel around, it seems I am on the opposite end of the tolerance. Small extension and large upper. Maybe I should by a billet to hopefully get a tighter fit.
Keep The Change Wrote:I'm afraid I will need to bed as well.
Just from moving the barrel around, it seems I am on the opposite end of the tolerance. Small extension and large upper. Maybe I should by a billet to hopefully get a tighter fit.
The CMT billet uppers weren't any tighter than the various brands of forged uppers I've used in the past. So, just going to a billet won't buy you anything without finding a specific brand that builds them tighter. Sounds like Mega used to, and now they don't. Of course, if the loctite bedding does it's job, it may not matter.
I may take one of my blaster AR's and bed the extension on it. Then, try to shoot it loose. If it stands up to multiple cycles of the smoking barrel test, I'm gonna quit worrying about it.
Keep The Change Wrote:I'm afraid I will need to bed as well.
Just from moving the barrel around, it seems I am on the opposite end of the tolerance. Small extension and large upper. Maybe I should by a billet to hopefully get a tighter fit.
The Mega forged uppers are by far the best fitting I have used, in fact one barrel required freezing the barrel, and using a heat gun on the upper to get it in. I'm sure their billets are made the same way. I won't buy anything else anymore.
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Thanks for the info Mike and Jurassic.
I believe AA beds the extensions with loctite, so it must work well. I'm just nervous doing it and getting loctite on crap that it shouldn't be on and then the issue with removal if need be.
Keep The Change Wrote:Thanks for the info Mike and Jurassic.
I believe AA beds the extensions with loctite, so it must work well. I'm just nervous doing it and getting loctite on crap that it shouldn't be on and then the issue with removal if need be.
This is an OK video. Don't forget to grease your threads before you install the barrel nut. I would also check inside the receiver and make sure none oozed out there. So far, that hasn't happened on the ones i've done.
http://www.youtube.com/watch?v=Wgw6MhO83Fg
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