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Thread: Formerly reliable BCM 11.5" upper has sudden cycling issues

  1. #11
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    If the gas tube is slightly misaligned, it can rub the tip where it interfaces with the gas key. I'd start by looking at it next to a new one. It's fairly cheap to replace so that shouldn't be a big issue. Also make sure to adjust the new one so that it interfaces with the gas key on the BCG without any drama.
    Life's a risk, enjoy the adventure - HOPLOETHOS

  2. #12
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    Look at the top of the gas tube and where it interfaces with the carrier key. If it is completely worn down, then that could be the issue assuming that all other issues have been checked.

    gas tube comparison.jpg



    Owner/Instructor at Semper Paratus Arms

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  3. #13
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    I bought a new gas tube. I'll try and get to the range tomorrow. The major difference is the flange at the end of the tube that meets the gas key on the carrier. The old one (lower) is worn smooth. I'll see if that has any effect on function.



    The gas port hole is eroded a tiny bit as well. Not sure if that is the culprit yet.


  4. #14
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    If there is significant wear at the bell, that's likely your culprit. I have a 12.5" upper that experienced the same issues as the OP. Replaced the gas tube and it was back to normal.

    FWIW, there is a gas tube "No-Go" gage available through Brownells that will tell you if your gas tube is worn. It is somewhat spendy though. Unless you were an armorer, you would probably be better off just buying a replacement tube.

  5. #15
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    Take a Mic (or calipers) and measure the diameter of the bell at a couple different spots.
    Black River Tactical
    BRT OPTIMUM Barrels - 16" EXT, 11.5" CAR, 9" PDW
    BRT Covert Comps 7.62 + 5.56 + 9mm
    BRT MarkBlue Gas Tubes - BRT EXT and PDW Lengths
    BRT EZTUNE Gas Tubes - CAR and MID
    BRT MicroPin Gas Blocks .750" & .625"
    BRT MicroTUNE Adjustable Gas Blocks
    BRT CustomTUNE Gas Ports

  6. #16
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    I have personally replaced a dozen or so like that after other troubleshooting methods didn't work and all of them worked afterwards.

    Quote Originally Posted by 1911-A1 View Post
    I bought a new gas tube. I'll try and get to the range tomorrow. The major difference is the flange at the end of the tube that meets the gas key on the carrier. The old one (lower) is worn smooth. I'll see if that has any effect on function.



    The gas port hole is eroded a tiny bit as well. Not sure if that is the culprit yet.




    Owner/Instructor at Semper Paratus Arms

    Facebook: https://www.facebook.com/SemperParatusArms/

    Semper Paratus Arms AR15 Armorer Course http://www.semperparatusarms.com/cou...-registration/

    M4C Misc. Training and Course Announcements- http://www.m4carbine.net/forumdisplay.php?f=141

    Master Armorer/R&D at SIONICS Weapon Systems- http://sionicsweaponsystems.com

  7. #17
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    Yup, just replaced one the other day that was a little down on power.

    Steel case ammo wouldn't lock back with a loose hold.

    The bulb measured .1755" , which is about .004" under the minimum spec for new.

    I'm interested to know what the wear limit is where you see consistent failures.


    Quote Originally Posted by Iraqgunz View Post
    I have personally replaced a dozen or so like that after other troubleshooting methods didn't work and all of them worked afterwards.
    Black River Tactical
    BRT OPTIMUM Barrels - 16" EXT, 11.5" CAR, 9" PDW
    BRT Covert Comps 7.62 + 5.56 + 9mm
    BRT MarkBlue Gas Tubes - BRT EXT and PDW Lengths
    BRT EZTUNE Gas Tubes - CAR and MID
    BRT MicroPin Gas Blocks .750" & .625"
    BRT MicroTUNE Adjustable Gas Blocks
    BRT CustomTUNE Gas Ports

  8. #18
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    Good reminder to have a couple 3 spares in the kit.
    - Rhino

  9. #19
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    I replaced one a couple of years back that was being used by a local trainer. I had built the upper for him and he started having failures. I asked him the round count and he estimated it was about 10K.

    On the other end, I had one that had less than 1500 rounds on it, and it was failing. But there were other factors such as some gas block leakage.

    Quote Originally Posted by Clint View Post
    Yup, just replaced one the other day that was a little down on power.

    Steel case ammo wouldn't lock back with a loose hold.

    The bulb measured .1755" , which is about .004" under the minimum spec for new.

    I'm interested to know what the wear limit is where you see consistent failures.



    Owner/Instructor at Semper Paratus Arms

    Facebook: https://www.facebook.com/SemperParatusArms/

    Semper Paratus Arms AR15 Armorer Course http://www.semperparatusarms.com/cou...-registration/

    M4C Misc. Training and Course Announcements- http://www.m4carbine.net/forumdisplay.php?f=141

    Master Armorer/R&D at SIONICS Weapon Systems- http://sionicsweaponsystems.com

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