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Thread: Barrel installation "botchery"?

  1. #41
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    https://www.m4carbine.net/showthread...rease-accuracy

    Basically used to "bed" the barrel extension in the upper. Does not "glue" it in, just like bedding does not glue in the action in a bolt action rifle.
    In the link Rob Jennsen among others mention using this method in an accuracy build, unlike the situation presented by the OP.
    Only mentioned it because he described significant movement of the barrel extension in the upper prior to tightening barrel nut. This method just might help avoid the need to buy another upper, if it works.
    Not much to lose, maybe $10-15 on Amazon.

  2. #42
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    Another very interestting bit of info.

    I saw a reference to the 10/22 in that thread, and will mention that I've personally used AccraGlass with stainless dust embedded into it as a way to tighten up the barrel in a 10/22 receiver. Those don't get nearly as hot as an AR, so I don't suggest using it. But it was nice to see the same concept being used.

    To the OP: What is the outer size of the barrel exstenion and inside diameter of the upper at the barrel end? I wonder how much slop there really is. The numbers will tell.

  3. #43
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    Thanks.

  4. #44
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    Quote Originally Posted by cjb View Post
    Another very interestting bit of info.
    To the OP: What is the outer size of the barrel exstenion and inside diameter of the upper at the barrel end? I wonder how much slop there really is. The numbers will tell.
    A fine question, sir.

    inside diameter of the upper at barrel end x-axis: 0.996 in
    inside diameter of the upper at barrel end y-axis: 0.996 in

    barrel extension x-axis: 0.990 in.
    barrel extension y-axis: 0.990 in.

    I'm additionally taking 2 measurements for each diameter ("y-axis" is top to bottom and "x-axis" is side to side) to address previous considerations in warpage from excessive tightening.

  5. #45
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    The wiggle isn't due to tolerance stacking, it's due to one or both parts being out of tolerance. Since two barrels have had the same problem, my guess is it's the upper.

    Green Loctite won't fill the gap on a barrel extension to upper receiver fit if it wiggles. It only works if the fit between the parts is tight
    Last edited by MistWolf; 01-20-14 at 22:31.
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  6. #46
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    Did you use the same gas tube on both builds?
    Train 2 Win

  7. #47
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    The blueprint spec on the barrel extension is .9987 -.0005 Diameter shall not exceed .999 after assembly, before finish is applied.

    Upper is 1.000 bore +-.002

    So lets say 1.002 and .9982 as the sizes.... just under .002 slop. Assuming the errors in gauging, you've got .006 slop.

    In fact, the OD is much easier to meaure, and the ID is a bit more difficult. You'd want a bore gauge for the upper, and a 1" micrometer, not calipers, for the barrel extension.

    Continuing with the thought process, its more likely if you used quality instruments, your .990 barrel extension is more correct, and is undersized. Can't say on the upper, but if it was in spec, you'd be talkin' .008 to .012 slop.

    Pause for thought.... dunno what to say.
    Last edited by cjb; 01-20-14 at 23:51.

  8. #48
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    Quote Originally Posted by T2C View Post
    Did you use the same gas tube on both builds?
    Different gas tube.

    Quote Originally Posted by cjb View Post
    ...Continuing with the thought process, its more likely if you used quality instruments, your .990 barrel extension is more correct, and is undersized. Can't say on the upper, but if it was in spec, you'd be talkin' .008 to .012 slop...
    I totally dig where you're going with this, but my equipment just isn't up to the cause. I only have inexpensive calipers. I did notice the measurement changed depending on what part of the "teeth" the calipers got a "bite" with. I tried to take the measurements from the same place on the calipers as I could, so they're consistent, but who is to say I took them at the right place for the most real number? Finally, I don't have anything known to the 1/1000th of an inch to calibrate/zero these calipers on. I just have to zero from "all closed".

    I've spent enough time troubleshooting complex systems to understand exactly how correct Iraqgunz is. If this isn't approached with controlled and methodical isolation, there will be much chasing of the tail--especially without tools to trace the system, and double especially without a necessary level of experience in the system. Great troubleshooting skills can only go so far before we're left branching out across multiple possibilities where experience would otherwise lead us.

  9. #49
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    Try putting the carbine upper on the rifle lower and shoot another group. Sometimes the difference in stockweld (cheek contact with the stock) between the rifle and carbine stocks can have an effect on accuracy, especially when using optics. Consistent placement of the eye behind the scope is very important when shooting groups.
    Train 2 Win

  10. #50
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    In reading the entire thread did I miss if you reused the old barrel nut or got a new one for the DD barrel? Barrel nuts are like crush washers use it once and do not reuse again.
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