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  1. #11
    Ark
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    I should be able to get out tomorrow and try it with a known good complete lower and a 1,000ish round known good BCG. Results of that should hopefully tell me what direction to go.

  2. #12
    Master Brad69's Avatar

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    Buffer is the problem!

    I bet you 10 of CMs cheese burgers!
    U.S. Army retired

  3. #13
    I still care....Really
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    Quote Originally Posted by Brad69 View Post
    Buffer is the problem!

    I bet you 10 of CMs cheese burgers!
    I will see your bet and raise it 2 deluxe bacon CB's....

    Could be the buffer.
    AKA..Thor. Odin son. God of thunder.
    But you can call me John.....Force.

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  4. #14
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    I have fixed similar issues by replacing the buffer spring with a flat wire spring

  5. #15
    Master bcannon's Avatar

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    To answer your questions, a standard rifle buffer weighs 5 oz. Yes they do make lighter ones. Yes you can disassemble the buffer by driving out the roll pin, remove the rubber bump pad and dump out the weights. You can replace one of the weights or all if you want with lighter ones. I would drop your upper on a known fully functional lower like your 6920, easy way to see if its a lower issue. It could be a slightly tight chamber but I'm betting that its a slightly clogged gas port or tube. I'll bet 4 dbl bacon cheeseburgers on it. I picked up a new/old stock upper for cheap that had similar issues. Put a small hose on the tube in the upper and could blow thru it but it took disassembling it to find thickened oily substance in the tube and gas block. Cleaned with a 12ga wire and a pipe cleaner, reassembled and had no issues after that. Good luck finding the issue.

  6. #16
    Ark
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    Swapped lowers and tested. Rifle upper runs fine on the carbine lower, carbine upper runs fine on the rifle lower with one failure to lock open.

    Looking like an issue with the lower. The carbine upper is gassed more and can run on it, but the rifle upper just can't. The rifle upper can run on a carbine buffer and spring.

    I think the next step is to try to lose about 2oz from the rifle buffer. I saw some people have replaced the weights with a section of aluminum rod to retain at least some anti-bolt bounce capability. I may also pull the gas tube and clean it out with some wire or something just to be sure.

  7. #17
    Master bcannon's Avatar

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    The buffer weights are pretty cheap, I'd buy premade to help reduce possible issues.

  8. #18
    Grandmaster SmileDocHill's Avatar

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    Any idea as to whether it has been used with a .22 conversion kit? That will support junk accumulation in the gas tube more than normal. It's my understanding the gas tube historically is not otherwise prone to buildup. (?) Not that it can't happen, not that I wouldn't check that as a possible source of the bottleneck, just saying for conversation sake so other more informed members can correct my logic.
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  9. #19
    Ark
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    Six weights total, all weigh out at .64 oz, indicating steel and not tungsten.

    Aluminum weights are about .22 oz.

    .42 oz weight savings per weight changed, maximum I can remove is 2.52 oz without taking weights out altogether. That should reduce the rifle buffer weight to at or below the weight of a standard carbine buffer.

    Quote Originally Posted by SmileDocHill View Post
    Any idea as to whether it has been used with a .22 conversion kit? That will support junk accumulation in the gas tube more than normal. It's my understanding the gas tube historically is not otherwise prone to buildup. (?) Not that it can't happen, not that I wouldn't check that as a possible source of the bottleneck, just saying for conversation sake so other more informed members can correct my logic.
    I kinda doubt it. The upper is LE surplus and was probably dumped in storage long before those CMMG conversions became a thing, But, it seemed to run great on the carbine lower even on a dog's lunch magazine of low pressure steel case.

    Last edited by Ark; 12-13-2019 at 11:27.

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