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Thread: Undergassed?

  1. #1
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    Undergassed?

    Before I start, I purchased the rifle in question long before I undertood the differences between makers. So any responses to this effect is unecessary. This carbine has been relegated to a loaner rifle and will not ever be used for SHTF or formal training.

    Many years ago I purchased a mid-length carbine for a song. As it turns out there is a reason for the low price. It's a 14.5" CMMG with a permanenetly attached "long" Flash Hider on a Bushmaster Lower.

    It short strokes when I shoot anything but "hot" ammo (ie: 5.56 pressure rounds). I've already checked the Gas Key and replaced the Gas Rings. I don't see any leaks around the pinned Gas Block.

    What happens is even with crappy 55-grain Wolf WPA it cycles fine but will not lock back on the lasst round. I've tried different magazines with the same results. Like I said, hotter rounds do fine. I have also tried dfferent BCGs, buffer weights (upt to H3) and a new spring.

    I was thinking of replacing the Gas Tube next. Any other suggestions? Thanks in advanced.
    Last edited by CarlosDJackal; 08-14-12 at 15:42.
    We must not believe the Evil One when he tells us that there is nothing we can do in the face of violence, injustice and sin. - Pope Francis I

  2. #2
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    Changing the gas tube will likely do little to change things unless there's a problem with the existing tube.

    You might have the gas port openned a little.... or at least checked to se how big the port is.
    "You people have too much time on your hands." - scottryan

  3. #3
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    1. Make sure that the FSB is straight and not blocking the port.

    2. CMMG is known to be overgassed. So this is not the norm.

    3. Worst case scenerio is I would pull the gas block and measure the port. I believe it should be around .063 or so.



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  4. #4
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    Is the gas block pinned or clamped to the bbl? The reason I ask is that I recently picked up a used AR with similar issues. Turns out that it had sustained an impact to the clamped front sight/gas block, causing a slight misalignment. No evidence of gas leakage around the block/bbl interface.

    Removed the gas block & cleaned everything thoroughly. Reinstalled, using a bubble level to make sure gas block was level with upper receiver. It now runs with everything I put through it.

    Sent from my Sprint EVO using Tapatalk 2

  5. #5
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    My brother had problems with an AR like this before. He changed the buffer to something lighter, sorry can't remember the weight or brand. It fixed his problem and is cheaper than most fixes.

  6. #6
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    Quote Originally Posted by Iraqgunz View Post

    3. Worst case scenerio is I would pull the gas block and measure the port. I believe it should be around .063 or so.
    ^Isn't that the number for a 14.5 CAR?

    OP has a 14.5 MID.


    My guess is gas port blocked by FSB or burr or simply too small.

    Also verify the bolt face retracts to a position ~.250" behind the bolt catch, to rule out any weird out of spec parts.
    Last edited by Clint; 08-14-12 at 19:00.
    Black River Tactical
    BRT OPTIMUM HFCL Barrels - Hammer Forged Chrome Lined 11.5", 12.5", 14.5"
    BRT OPTIMUM Barrels - 16" MPR, 14.5" MPC, 12.5" MRC, 11.5" CQB, 9" PDW
    BRT EZTUNE Preset Gas Tubes - CAR and MID
    BRT Covert Comps 7.62, 5.56, 6X, 9mm
    BRT MarkBlue Gas Tubes - BRT EXT, EXC and PDW Lengths
    BRT MicroPin Gas Blocks - .750" & .625"
    BRT MicroTUNE Adjustable Gas Blocks
    BRT CustomTUNE Gas Ports

  7. #7
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    You're probably right. I want to say maybe .068 or so. Off the top of my head I am not sure.

    Quote Originally Posted by Clint View Post
    ^Isn't that the number for a 14.5 CAR?

    OP has a 14.5 MID.


    My guess is gas port blocked by FSB or burr or simply too small.

    Also verify the bolt face retracts to a position ~.250" behind the bolt catch, to rule out any weird out of spec parts.



    Owner/Instructor at Semper Paratus Arms

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  8. #8
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    I nevr thought of a misaligned Gas Block. I'll have to take a closer look. Thanks all!!
    We must not believe the Evil One when he tells us that there is nothing we can do in the face of violence, injustice and sin. - Pope Francis I

  9. #9
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    Let us know what you discover. I know I've learned a lot in diagnosing my carbine's issues.

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  10. #10
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    Quote Originally Posted by Iraqgunz View Post
    I would pull the gas block and measure the port. I believe it should be around .063 or so.
    Quote Originally Posted by Iraqgunz View Post
    You're probably right. I want to say maybe .068 or so. Off the top of my head I am not sure.
    Still too small.
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

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