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Thread: First BCM upper build, possibly my last

  1. #11
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    That groove really isn't for the hand guard cap, it's just an under cut to remove any tool radius at the corner of the gas block journal and the shoulder. With a hand guard cap the rear of the gas block or front sight typically sits about .026"-.030" from the shoulder with the hand guard cap filling that space. Without a hand guard cap expect to see about .030" gap between the shoulder and the gas block. I'd be concerned with the rail/barrel nut interface, if the gas block is inline enough for the gas tube to line up with the carrier key without noticeable interference with the carrier key or barrel nut, I'd say its where it needs to be. If you can slide on the gas block without the aid of a hammer I'd expect to see some gas leakage at first.

    What direction does the gas block appear to be canted?

  2. #12
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    Quote Originally Posted by JBowles View Post
    That groove really isn't for the hand guard cap, it's just an under cut to remove any tool radius at the corner of the gas block journal and the shoulder.
    The relief cut on the OP's barrel is interesting. I have two DD CHF barrels purchased about a year apart and neither of them has a relief cut at the shoulder.



    Quote Originally Posted by JBowles View Post
    With a hand guard cap the rear of the gas block or front sight typically sits about .026"-.030" from the shoulder with the hand guard cap filling that space. Without a hand guard cap expect to see about .030" gap between the shoulder and the gas block.
    The gap, or lack thereof, is determined by the gas block manufacturer. Noveske/Vltor gas gas blocks are designed to be installed flush with the shoulder. Even if you offset a gas block that isn't designed to be by 0.030, given a plenum diameter of 0.162+-, you will still have about 0.020 before the barrel will occlude the plenum in the gas block.

  3. #13
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    Albeit a small sample size, but I've built a handful of uppers (~5) and I've always snugged the gas block up to that groove with no "gap". Every one of these uppers has functioned perfectly. The combinations have included BCM barrels with BCM gas blocks, Centurion barrel with Vltor gas block, and DD barrel with DD gas block.

    So I learned something new today, but fortunately for me the hole sizes are large enough that my gas systems are working fine.

  4. #14
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    I measure from the center of the hole in the gas block to the end and from the shoulder of the barrel to the center of the gas port. Then use the appropriate feeler gauge against the shoulder as I slide the gas block home. Probably unnecessary, but it's simple and easy to do.. Ron

    PS: Some of my barrels had the cut and some didn't, but they all needed the feeler gauge..
    Ain't no pockets on a shroud..

  5. #15
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    Quote Originally Posted by Kokopelli View Post
    I measure from the center of the hole in the gas block to the end and from the shoulder of the barrel to the center of the gas port. Then use the appropriate feeler gauge against the shoulder as I slide the gas block home. Probably unnecessary, but it's simple and easy to do.. Ron

    PS: Some of my barrels had the cut and some didn't, but they all needed the feeler gauge..
    Whose gas blocks were you using?

  6. #16
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    Troy and MI.. Ron

    Quote Originally Posted by MK18Pilot View Post
    Whose gas blocks were you using?
    Ain't no pockets on a shroud..

  7. #17
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    Here is a larger version of the barrel (before I had it pinned).

    Click on pic for full version

    Last edited by danpass; 08-02-13 at 12:40.
    Dan Miami, FL

  8. #18
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    Reading is fundamental...

    I thought the OP's barrel was a DD. I now see it is a BCM and that explains the relief cut.

  9. #19
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    Quote Originally Posted by MK18Pilot View Post
    The relief cut on the OP's barrel is interesting. I have two DD CHF barrels purchased about a year apart and neither of them has a relief cut at the shoulder.





    The gap, or lack thereof, is determined by the gas block manufacturer. Noveske/Vltor gas gas blocks are designed to be installed flush with the shoulder. Even if you offset a gas block that isn't designed to be by 0.030, given a plenum diameter of 0.162+-, you will still have about 0.020 before the barrel will occlude the plenum in the gas block.
    Yeah exactly, I should of mentioned that, generally gap(<.030") or no gap you should be good to go.

    As for the relief cut, a relief vs a radius both are acceptable as long as they are kept small.
    Last edited by JBowles; 08-02-13 at 12:48.

  10. #20
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    It is easier to buy an assembled and test fired upper than do it yourself unless you want to buy the proper equipment.

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