Barrel bedding AR, Loctite 620

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  • lazarus

    Ultimate Member
    Jun 23, 2015
    13,678
    Or at least I think 620 is the one recommended for bedding an AR barrel in to the upper receiver. Just how difficult is it to remove later? Are we talking fashioning a wooden dowel and using a dead blow hammer to get it loose later? Or just some elbow grease?

    I am trying to determine how permanent it is as it makes me nervous to install a barrel in a receiver were replacing the barrel means effectively replacing the receiver also (or having to go to great lengths to get it loose).

    Been working to accurize a build and I think it is an issue with the barrel, not the fitment. But I figure before going and tossing the barrel and getting a new one, I should do some work to make sure it isn't something else. Lapped the receiver and I need to get gauges (it SEEMS to be headspacing correctly looking at the brass). Group sizes are still rather large and I don't recall the barrel being loose in the upper, but figure it can only help.

    Thanks.
     

    Doco Overboard

    Ultimate Member
    What does the loc-tite do? I don't work on Ar's so I'm trying to learn something here. When you lap, how are the upper threads treated and how do you ensure the barrel stub is perpendicular to the upper threads and lock nut or method of fastening the barrel to the upper.
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    If you have a padded vise and a plastic mallet, just slight taps will drive the upper off the barrel.

    People make way too much fuss over it being permanent. It's not.


    Loctite 620 is the higher temp rated sleeve retainer and is what I use.

    Using stainless Shims does produce better results but is MUCH harder to work with, so much so that I don't teach it in my classes.
     

    lazarus

    Ultimate Member
    Jun 23, 2015
    13,678
    If you have a padded vise and a plastic mallet, just slight taps will drive the upper off the barrel.

    People make way too much fuss over it being permanent. It's not.


    Loctite 620 is the higher temp rated sleeve retainer and is what I use.

    Using stainless Shims does produce better results but is MUCH harder to work with, so much so that I don't teach it in my classes.

    Thanks! Time to order some 620 and redo before I get to the range next time.

    Doco for the lapping, the tool is a tight fit in the upper’s extension hole. So long as the upper extension slot/hole itself isn’t canted it should keep the lapping pretty darned concentric.

    For the threads you don’t really need to do anything so long as you don’t go overboard with lapping compound. Very little got on the threads when I did it and wiped off easy. Apply some aeroshell and out on the barrel and barrel nut.

    The loctite takes up some of the slop between the barrel extension and the upper receiver so that it can’t cant as the gun is firing and the bolt is unlocking and then slamming back in to lock. Or at least reduces the ability for it to possibly twist or cant.
     

    Doco Overboard

    Ultimate Member
    The loctite takes up some of the slop between the barrel extension and the upper receiver so that it can’t cant as the gun is firing and the bolt is unlocking and then slamming back in to lock. Or at least reduces the ability for it to possibly twist or cant.

    I see now, its so the barrel extension is supported when that barrel nut is tightened. What I'm asking is that even if the face of the upper/receiver is perpendicular, if the threads are not exactly square on the upper at some point around the circumference of the extension there will be more interference so to speak.

    When the upper is lapped to improve the face of it, is there a way to true the threads slightly at the same time or does that even matter? Like a lapping tool that has a die on it.

    I quick looked on yo tube and seen where someone used green loctite, wiped a lot of it off because there is already a pretty close fit but thought if the threads aren't square there still could be unequal radial tension to the barrel extension.

    Could be too that the area of the threads is not great enough to overcome the leverage created by the lapped extension/upper surfaces once they are improved as well, just because they're clamped together tightly. I don't know.
    One host indicated torque specs ranging from 30-40 up to 80ftlbs when installing the barrel lock nut to facilitate the remainder of assembly steps, two or more others bumped the extension in increments to get what they needed even before trying what they had was sufficient to finish the job. Only the one that I was able to view used the green and he turned it right up checking with a gas tube gauge and with out bumping.

    Maybe he was just the most prepared for making the video or the most experienced assembler I'm not sure. The whole thing is interesting to me now.
     

    SWO Daddy

    Ultimate Member
    Jun 18, 2011
    2,468
    I've never felt the need to do this before, but why wouldn't you use Loctite 640 for this application? That's what's used for automotive cylinder sleeves and seems more analogous to this use.
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    I've never felt the need to do this before, but why wouldn't you use Loctite 640 for this application? That's what's used for automotive cylinder sleeves and seems more analogous to this use.

    Because the gaps required to fill the void in the B.E. to Receiver Interface often exceed the specs for 640. 620 has a higher temperature rating.

    Loctite 640
    Temperatures up to 400ºF
    Gap Fill: 0.007" Maximum

    Loctite 620
    Temperatures up to 450ºF
    Gap Fill: 0.015" Maximum
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    I see now, its so the barrel extension is supported when that barrel nut is tightened. What I'm asking is that even if the face of the upper/receiver is perpendicular, if the threads are not exactly square on the upper at some point around the circumference of the extension there will be more interference so to speak.

    When the upper is lapped to improve the face of it, is there a way to true the threads slightly at the same time or does that even matter? Like a lapping tool that has a die on it.

    I quick looked on yo tube and seen where someone used green loctite, wiped a lot of it off because there is already a pretty close fit but thought if the threads aren't square there still could be unequal radial tension to the barrel extension.

    Could be too that the area of the threads is not great enough to overcome the leverage created by the lapped extension/upper surfaces once they are improved as well, just because they're clamped together tightly. I don't know.
    One host indicated torque specs ranging from 30-40 up to 80ftlbs when installing the barrel lock nut to facilitate the remainder of assembly steps, two or more others bumped the extension in increments to get what they needed even before trying what they had was sufficient to finish the job. Only the one that I was able to view used the green and he turned it right up checking with a gas tube gauge and with out bumping.

    Maybe he was just the most prepared for making the video or the most experienced assembler I'm not sure. The whole thing is interesting to me now.

    If the upper receiver face is not square to the bore, the barrel will sit off axis to the Picatinny rail and sights. Squaring the face trues the alignment. I have worked on thousands of AR's that the sights had to be maxed out to get POA/POI. Truing the upper receiver face is the most common remedy.

    The bedding prevents slop between the upper receiver and barrel extension. So does shimming and buying barrels with oversized B E.'s. Bedding/Shimming or thermal fitting prevents the barrel from shifting from thermal variables and recoil forces. Watch a high speed video of an AR firing, and look at the barrel. The barrel moves a LOT under the firing sequence. Preventing it from shifting under thermal variances and recoil helps consistency.

    In regards to the Barrel Nut and Threads being true, there is enough play in the Barrel Nut threads to evenly load the Barrel Extension Crush Shoulder to the Upper Receiver. The only truing needed on the Upper or Barrel Nut threads is to ensure they pass hand fitting.

    If you dont believe any of this call Criterion, they have a video talking about this very phenomenon and teach people how to do it on their YouTube Channel.

    Joe Carlos also has a very good video on YouTube about it. Again if you dont believe it contact him.

    The AMU also does upper receiver truing and bedding/shimming/thermal fitting of barrels.
     

    Doco Overboard

    Ultimate Member
    If the upper receiver face is not square to the bore, the barrel will sit off axis to the Picatinny rail and sights. Squaring the face trues the alignment. I have worked on thousands of AR's that the sights had to be maxed out to get POA/POI. Truing the upper receiver face is the most common remedy.

    The bedding prevents slop between the upper receiver and barrel extension. So does shimming and buying barrels with oversized B E.'s. Bedding/Shimming or thermal fitting prevents the barrel from shifting from thermal variables and recoil forces. Watch a high speed video of an AR firing, and look at the barrel. The barrel moves a LOT under the firing sequence. Preventing it from shifting under thermal variances and recoil helps consistency.

    In regards to the Barrel Nut and Threads being true, there is enough play in the Barrel Nut threads to evenly load the Barrel Extension Crush Shoulder to the Upper Receiver. The only truing needed on the Upper or Barrel Nut threads is to ensure they pass hand fitting.

    If you dont believe any of this call Criterion, they have a video talking about this very phenomenon and teach people how to do it on their YouTube Channel.

    Joe Carlos also has a very good video on YouTube about it. Again if you dont believe it contact him.

    The AMU also does upper receiber truing and bedding/shimming/thermal fitting of barrels.

    Never said I didn't believe it, just thinking about and responding to the op's original question.

    I also do not see the need Mr. Carlos just to re-affirm your statement that you agree with him, that's good enough me and all I really need to hear.

    Again, I don't need a basis or further substantiation from the AMU, Mr. Carlos or CBI who are not currently part of this conversation to ask a legitimate question. Especially one that I find particularly interesting and wish to participate in.

    However,thanks you for your insight regarding the fitting of the barrel nut. That makes sense and is a very simple answer to what I initially perceived as a potentially difficult problem for some one not familiar with the weapon type.

    Have a nice day! and thanks for your reply.
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    Never said I didn't believe it, just thinking about and responding to the op's original question.

    I also do not see the need Mr. Carlos just to re-affirm your statement that you agree with him, that's good enough me and all I really need to hear.

    Again, I don't need a basis or further substantiation from the AMU, Mr. Carlos or CBI who are not currently part of this conversation to ask a legitimate question. Especially one that I find particularly interesting and wish to participate in.

    However,thanks you for your insight regarding the fitting of the barrel nut. That makes sense and is a very simple answer to what I initially perceived as a potentially difficult problem for some one not familiar with the weapon type.

    Have a nice day! and thanks for your reply.


    There is a history between you and I, in regards to the AR and the advice I give here on MDS. I was simply offering alternatives in the chance you didn't believe what I was posting.

    You have a good day as well.
     

    atblis

    Ultimate Member
    May 23, 2010
    2,011
    Another option is to get an upper that fits tighter. I've found the BCM uppers to have a nice tight fit. They're intentionally made that way.
     

    teratos

    My hair is amazing
    MDS Supporter
    Patriot Picket
    Jan 22, 2009
    59,775
    Bel Air
    Another option is to get an upper that fits tighter. I've found the BCM uppers to have a nice tight fit. They're intentionally made that way.

    They are difficult to lap because you have to put them in the oven to get the lapping tool to fit, and lap quickly. That said, I use BCM uppers, and am happy with them.
     

    lazarus

    Ultimate Member
    Jun 23, 2015
    13,678
    Another option is to get an upper that fits tighter. I've found the BCM uppers to have a nice tight fit. They're intentionally made that way.

    Good to know, thanks!

    Eventually I’ll build a full on precision upper. Right now I’ve got a 20” HBAR with a stoner SS 5R 1:8 barrel that shoots 1MOA without any accurizing. I’ve got a classic firearms (they use BCA as the contractor according to them when I contacted them) 20” SS 5R fluted 1:8 HBAR barrel I am going to try in that upper, bed and lap it. My 20” isn’t strictly a bench rifle and the 4 or 5oz the fluting sheds is meaningful to me. If accuracy improves great, if it suffers back to the old barrel.

    My 18” 6.5 grendel upper has also not been accurized and it shoots slightly under 1MOA with a fluted, nitrided Classic Firearms HBAR barrel. It’s my deer rifle.

    My 16” .223 upper shoots 3-6MOA and I am working to accurize it. I am pretty sure the barrel has an issue, as I’ve ruled out most everything else. But I am not in a position to get another 16” barrel particularly soon, so I figure I’ll clean it up a little and see if it improves at all. If it’s still junk I am mostly out some time and some loctite that I’ll use on other projects (for what’s left in the bottle, which should be a lot of guns).

    I don’t need my 16” to be a sub MOA rifle, but I’d like to at least be pushing 2MOA with match ammo and no worse than 3MOA with commodity .223/5.56 55gr FMJ. Right now it’s 3MOA with the right match ammo and 5-6MOA with commodity.
     

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