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Thread: Precision rifle alternating between double feeds and short stroking-Troubleshoot how?

  1. #1
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    Question Precision rifle alternating between double feeds and short stroking-Troubleshoot how?

    I have an upper (sitting on a Gen2 Chainsaw lower I built) that has been giving me fits - short stroking across every buffer combination I've tried, but throwing double feeds almost as frequently. I've only had limited range time, and only taken it out four times, totalling less than 200 rounds through this thing, but I can't find a buffer combination that can make the short stroking and failure to lock back really go away, nor can I get through a dozen rounds without some sort of failure to feed.

    So far I've tried: H2 Buffer, Springco Blue Spring; H Buffer/Sprinco Blue; CAR Buffer Sprinco Blue; H-2/stock spring, CAR/Stock Spring; A5H2, A5H3 with stock VLTOR spring on the buffer/spring front. Still getting failures to lock back even with the CAR/Stock buffer/spring setup running IMI and PPU M193, and still getting double feeds in magazines which have ran otherwise faultlessly.

    Only other part of note is that sometimes the BCG feels a bit stick while unlocking - at first I assumed this was a breaking in process, as this is a match barrel, but it's persisted, and is somewhat inconsistent. I later noticed that the extractor pin at some point had protruded out a bit, but seems to have been sheared or pressed back enough that it doesn't seem to interfere - still no issues with ejections (makes very nice neat piles... when it decides to run in semi). Subbed in a different BCG, got a couple short strokes and gave up on that idea.

    I'm inclined to think maybe gas tube, or tolerance stack with that pin dragging slightly and the tight chamber as well as bolt lug friction, but outside of visual inspection there's not a ton I can do to look at that. The double feed also seems perplexing - this lower feeds other uppers just fine using the same magazines, though a fraction of these are probably just me reflexing a tap rack bang when I get a soft trigger because the bolt is caught halfway on the way back.

    What I'm at a loss for is if there's any more user end troubleshooting I should really look at, or should I just stop wasting time and ammunition and most likely send the complete upper back to where I ordered it from? Everything I've tried points towards that, though I may not have the round count to really say that the lower itself is GTG, I've just never had to troubleshoot two simultaneous malfunctions for which I don't know of a common cause outside of crap grade rifles, and this hardly meets that description.
    عندما تصبح الأسلحة محظورة, قد يملكون حظرون عندهم فقط
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  2. #2
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    Why don't you give us more details about the upper? Gas system, etc... If you are unable to troubleshoot it, I suggest you send it back from whence it came and explain the problem to them and allow them the chance to fix it.



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  3. #3
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    How do you know it's a short stroke. Are you feeling a weak and slow cycle? I ask because it could be outrunning the mag's ability to feed the next round and appear to be a short stroke... but you should be able to tell by feel which it is.
    "You people have too much time on your hands." - scottryan

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    I had the same issues with my AR 10 when I first got it together. It was my gas block. Loose enough to move a bit when shooting, but it seemed tight when I grabbed it, so I dismissed it for a while. It's a Centurion gas block on a Raineer Ultra Match. I had a friend that owns a machine shop do something to fix it, but I don't remember at the moment. I can look if you think this might be it. It was either drill and pin, or drill and tap for a set screw, I just don't remember.

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    Quote Originally Posted by markm View Post
    How do you know it's a short stroke. Are you feeling a weak and slow cycle? I ask because it could be outrunning the mag's ability to feed the next round and appear to be a short stroke... but you should be able to tell by feel which it is.
    This. I've seen what I thought was a gas leak issue, that ended up being a hogged-out gas port issue.
    "That thing looks about as enjoyable as a bowl of exploding dicks." - Magic_Salad0892

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    Quote Originally Posted by Iraqgunz View Post
    Why don't you give us more details about the upper? Gas system, etc... If you are unable to troubleshoot it, I suggest you send it back from whence it came and explain the problem to them and allow them the chance to fix it.
    16" Douglas barrel, Midlength gas, Pinned LP gas block (I can't visually see any signs of a gas leak) Young MFG BCG - though it's quite solidly staked. The thought occurs to me that I had the lower with a slightly odd buffer/spring combination when I tried the other BCG, so I'll take a closer look at that, might still be the BCG.

    I'm confident it's short stroking because it started failing to lock back on the last round with every magazine I've thrown at it, and while it's a pretty heavy barrel with a brake affixed, I'm not feeling the buffer reach the back of the stock, and I've had a few FTF cases where it has picked up a round from the magazine but lacked momentum to finish chambering the round. I did run the H2/Blue combo to be more confident it wasn't just outrunning the mag spring, but I'm fairly sure it's just too little input energy.

    At such a low round count, with all brand new hardware, I'm not sure how anything wouldn't be an assembly issue, so I'll give it one more look over.
    عندما تصبح الأسلحة محظورة, قد يملكون حظرون عندهم فقط
    کله چی سلاح منع شوی دی، یوازي غلوونکۍ یی به درلود
    Semper Fi
    "Being able to do the basics, on demand, takes practice. " - Sinister

  7. #7
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    How new is that YOUNG? Those fukkers are tight when new and take a few hundred rounds to settle down. We've experienced this first hand. It should be pretty wet inside and around the carrier in the portion that's nearest the ejection port.

    And do you have a good old fashioned SS USGI Spring? Try to minimize the aftermarket soup sangwich effect as much as possible. The H2 should run fine. But maybe this gun likes a plain old H buffer.
    Last edited by markm; 02-18-14 at 13:16.
    "You people have too much time on your hands." - scottryan

  8. #8
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    Have you checked the wear on the gas tube where the gas key interfaces? Specifically, the raised shoulder of the gas tube where it interfaces with the gas key.

    Quote Originally Posted by TehLlama View Post
    16" Douglas barrel, Midlength gas, Pinned LP gas block (I can't visually see any signs of a gas leak) Young MFG BCG - though it's quite solidly staked. The thought occurs to me that I had the lower with a slightly odd buffer/spring combination when I tried the other BCG, so I'll take a closer look at that, might still be the BCG.

    I'm confident it's short stroking because it started failing to lock back on the last round with every magazine I've thrown at it, and while it's a pretty heavy barrel with a brake affixed, I'm not feeling the buffer reach the back of the stock, and I've had a few FTF cases where it has picked up a round from the magazine but lacked momentum to finish chambering the round. I did run the H2/Blue combo to be more confident it wasn't just outrunning the mag spring, but I'm fairly sure it's just too little input energy.

    At such a low round count, with all brand new hardware, I'm not sure how anything wouldn't be an assembly issue, so I'll give it one more look over.
    "That thing looks about as enjoyable as a bowl of exploding dicks." - Magic_Salad0892

    "The body cannot go where the mind has not already been."

  9. #9
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    Figured it out, I think. Carrier key. It's got slop, some rather asymmetric wear on the sides, and will move side to side and forwards/backwards. The two diagonally opposite corner friction surfaces of the key are shining - at 186 rounds down the pipe. The deformation on the bolts from the staking is still there, but with just a hex driver and pinky pressure I can get a little bit of rotation on the bolts, and the key itself will move too. Best guess - not torqued down enough prior to staking, but I haven't monkeyed with this enough to know any alternative causes.

    Does this make adequate sense?

    Mark, I've ran all three of those buffers with just the USGI spring - and I agree that the H2/USGI should have ran. Ran it hilariously wet, still same issues.
    Last edited by TehLlama; 02-18-14 at 13:34.
    عندما تصبح الأسلحة محظورة, قد يملكون حظرون عندهم فقط
    کله چی سلاح منع شوی دی، یوازي غلوونکۍ یی به درلود
    Semper Fi
    "Being able to do the basics, on demand, takes practice. " - Sinister

  10. #10
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    His gun is new. I've had that failure myself... but unless the tube was out of spec to begin with, I can't imagine it being worn out already.

    I need to check that out on the gun I had the problem with now that you mention it. I wanted to monitor the tube to see if the carrier key was eating up gas tubes for some reason.
    "You people have too much time on your hands." - scottryan

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