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Frankly

AR15 Criterion Barrel to Receiver ~ How Tight is Too Tight?

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So after watching and reading up on Accurizing AR-15s my latest build is attempting to incorporate some of the things I've seen. I'm using an Aero Precision enhanced upper with a mid-weight Criterion barrel. I've already lapped the front of the receiver using the Brownells lapping tool (I did just enough to show clean metal 360-degrees) and when I test fit the barrel it pushes in half way in before seizing. In fact I had to pound the barrel out from the opposite direction with a 1" dowel it was so tight. I know I can tap the barrel in with a rubber mallet but never having done this, I wonder how tight a "normal" barrel to receiver should fit? How tight is too tight that I'm going to have problems? 

 

I'm thinking that I should throw the barrel into the freezer so it contracts and heat up the end of the receiver so it expands and hopefully the barrel will slip in with minimal persuasion. But never having done this before I'd like to hear from people who have had tight barrel to receiver fits? 

 

Is it possible to get the connection between barrel and receiver too tight in a way I might crack or damage one or the other? 

 

Is tapping the barrel in from the muzzle with a piece of wood and a rubber hammer going to stress the barrel or swell out the muzzle end ever so slightly and de-accurize my rifle? I mean good 16p framing nail power with a 12 oz rubber hammer. 

 

I'm assuming that if it's this tight it's pointless to be adding Green Loctite or anything else to tighten the fit?

 

Or should just lube the barrel's receiver extension with Mobil One and shove it in, give it tap to send it home and stop being a worrywart?  

 

Sorry I know this sounds kind of obsessive but having never done it before I rather get some snark than to mess it up. 

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You could always measure them to make sure some thing isn't really off. But, when I am fortunate enough to get a fit that tight I heat the receiver gently and cool the barrel and tap them together exactly as you describe.  I usually only bother with bearing retaining compound (green loctite) when the fit is loose.

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Larue has a very good video on this. They make sure the barrel to receiver fit is only possible by heating up the receiver then quickly inserting the barrel before it cools. This is one of the things they do to make accurate rifles.

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Check out JP Rifles recent video on this. 

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Yeah the JP video and the Criterion videos got me going on this. Thanks

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I had to freeze my Odin works barrel and tap it into the aero enhanced upper. I didn't heat the upper though. I considered it a good thing. 

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I would heat the barrel with a heat gun, I suspect this will do it.  I would add the Loctite 609 because it will at 3000 PSI tensile strength and increase the rigidity of the interface.

 

Like the other poster suggested I also measure the OD of the barrel extension and the ID of the receiver.  They should be very close.

 

I am a believer in this technique obviously.

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On 4/14/2019 at 8:43 PM, 12glocks said:

I would heat the barrel with a heat gun, I suspect this will do it.  I would add the Loctite 609 because it will at 3000 PSI tensile strength and increase the rigidity of the interface.

 

Like the other poster suggested I also measure the OD of the barrel extension and the ID of the receiver.  They should be very close.

 

I am a believer in this technique obviously.

Why not loctite 620? Also, if you heat up the barrel wouldn't that make it even harder to get into an already tight upper?

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9 hours ago, 157Beck said:

Why not loctite 620? Also, if you heat up the barrel wouldn't that make it even harder to get into an already tight upper?

Yes heat up the receiver is what it should have said.

 

Loctite 609 is what was recommended by Criterion Barrel in their series on accurizing the AR.  I looked at 620 tonight and perhaps it is adequate but I don't know.

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I did the works... lapping, freezing barrel, heating receiver, even Loctite in spite of the already tight fit that needed to be tapped in the last 1/8”. Will shoot next week but hoping it’s awesome.

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On 4/25/2019 at 7:43 PM, 12glocks said:

Yes heat up the receiver is what it should have said.

 

Loctite 609 is what was recommended by Criterion Barrel in their series on accurizing the AR.  I looked at 620 tonight and perhaps it is adequate but I don't know.

I'll have to switch to 609 from now on. Thanks

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6 hours ago, 157Beck said:

I'll have to switch to 609 from now on. Thanks

620 makes more sense for barrel installation as its rated for higher temperature than 609, I would think.

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On 4/26/2019 at 1:37 AM, Frankly said:

I did the works... lapping, freezing barrel, heating receiver, even Loctite in spite of the already tight fit that needed to be tapped in the last 1/8”. Will shoot next week but hoping it’s awesome.

Let us know how it shoots!

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2 hours ago, mpom said:

620 makes more sense for barrel installation as its rated for higher temperature than 609, I would think.

Perhaps, unless you want to break it free but you could use a torch.

On a whim I tired blue loctite (Not sure which one, it's the new oil resistant variant I believe) on a barrel installation and the gun shot great.  I tried to remove the barrel out of the upper and that sucker is in there big time.  I have not shot any 70 round stages with the gun though.  Looking at the JP info on thermal dissipators makes me think most of the heat is well in front of the receiver extension, which is logical.

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I just built a new upper and lapped the receiver, used 620 and tightened everything up.  There was a little play in the upper and lower so I just used aluminum foil to make a shim and it’s tight now.  Shot it yesterday with my Fenix 55 grain hoser and it shot a .64” group at 100  Yrds for a 5 shot group.  I don’t know if it all made much of a difference due to not shooting it before lapping or using the 620 but I’d like to think it helped some.  

 

This was with a Stretch barrel, Vltor upper receiver and Slr handguard.  

DD762A41-0D76-44E0-B2B8-5132C9C59C7A.jpeg

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