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dghboy315

loose barrel to upper fit?

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I'm trying to build a precision ar15 so I got a 20" WOA barrel and a Vltor MUR upper. Both high quality parts according to my research. The issue is when I insert the barrel into the upper there is some play. It slips in very easily. When it is in, there is a little rattle when the whole unit is shaken. The barrel nut has not been installed. I have never experienced this in the 6 other ar-15s I have built.

Is this a problem? Will this effect accuracy? Should I shim it or use Loctite? Should I exchange the upper for something else?

I have not torqued the barrel nut yet, but I'm assuming the rattle will go away when I do it. I was just wondering if that little bit of space will negatively effect the rifle.

Edited by dghboy315

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I shimed mine measure the inside diameter of your upper then subtract the outside diameter of the barrel. Ithen cut the shim material tofit around the barrel. Put the barrel in the freezer for a few hours hit the upper with a heat gun then installed the barrel with the shim and got a very tight barrel to upper fit.

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I shimed mine measure the inside diameter of your upper then subtract the outside diameter of the barrel. Ithen cut the shim material tofit around the barrel. Put the barrel in the freezer for a few hours hit the upper with a heat gun then installed the barrel with the shim and got a very tight barrel to upper fit.

what shim material did you use?

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Either your upper is out of spec or you barrel extension is. I would not shim. Have both checked and return the one that is. I would at least try a couple of receivers to get a good fit.

Get your receiver trued and squared and don't shim.

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I used .0015 shim stock and if you measure 6 different uppers your going to get 6 different measurements. I know a manufacturer of highend ar's that uses a simular technique.

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Yup, my upper from a "major manufacturer" also had a shimmed barrel. If it's ok for the big boys probably ok for us normal folks.

Doug

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I have found the Vltors to be a bit on the loose side and have had to shim several of them. A very tight, or interference fit is preferable.

If the fit is loose, you very well have some accuracy issues, and it is highly unlikely that anything is out of spec.

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Thanks guys. I'm leaning towards return the Vltor upper to brownells. I have not decided on whether to exchange it for another Vltor MUR or return it for a different upper. Opinions? Anyone have recommendations on a high precision upper?

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I went down this path, the VLTOR upper I got was not only oversize, it was also out of round, I've returned it, bought a JP upper AND shimmed it.

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Believe the shim bison sells is for timing the barrel nut, not for eliminating play betweeen extension and upper. Loctite makes products for this application, think they are sleeve retaining compounds.

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You are

Believe the shim bison sells is for timing the barrel nut, not for eliminating play betweeen extension and upper. Loctite makes products for this application, think they are sleeve retaining compounds.

Yes, you are correct. My mistake.

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I have brain farts all the time, especially while on the clock, dammit :)

Correct name is "retaining compound". An example is 609, bought mine from Amazon. Hope its not too difficult to remove as I am going to rebarrel an upper.

Will try some UnCure if needed.

Edited by mpom

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The BCM upper is made to have a tight barrel fit. The 3 I used to date indeed were tight, needed a few taps on the barrels to get them to seat.

Edited by PacMan

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The BCM upper is made to have a tight barrel fit. The 3 I used to date indeed were tight, needed a few taps on the barrels to get them to seat.

It depends. I had two BCM uppers side by side. One tight, one really not. They are always tight on BCM barrels, which I think have oversize barrel extensions.

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I wouldn't want to have to shim a new barrel. Dissimilar materials will expand/contract at different rates due to heat. That's bound to have an effect on your accuracy, especially at distance.

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I wouldn't want to have to shim a new barrel. Dissimilar materials will expand/contract at different rates due to heat. That's bound to have an effect on your accuracy, especially at distance.

There are already plenty of dissimilar metals in play, usually three. :)

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So I went against MarkCO's recommendation (I usually don't) and used some Loctite 638 sleeve retaining compound to solve my sloppy MUR receiver to extension fit problem. The rifle is tack driver and I don't see any thermal drift after longer strings of fire.

I may never get it apart again but shouldn't every new barrel deserve a new upper receiver? LOL

Edited by Colodrew

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