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Thread: What are some possible causes of these failures?

  1. #1
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    What are some possible causes of these failures?

    The Failures: The gun is basically a single shot.

    1. Chamber a round. Fire it.
    2. Bolt does not move, but maintains its grip on the cartridge with the extractor.
    3. Charging handle won't budge, making mortaring necessary to extract/eject spent case.
    4. Release the bolt, chambering the next round.
    5. Repeat at step 1.

    The above sequence happened on the very first round and on EVERY round fired thereafter. On a FEW rounds, the cartridge/bolt would half-way retract out of the chamber.

    I think I've ruled out magazines and ammo due to the fact that it choked on every mag I had, and with TWO types of ammo.

    Below is the configuration info for the gun/mags/ammo

    The Upper: LMT 14.5" w/ perm. attached Vltor VC-1 (done by ADCO). Aimpoint T1 in Larue mount. Upper has never been fired before today.
    The BCG: FA BCG & bolt with McFarland single piece gas ring. (unknown maker)
    The Lower: LMT-made DSA lower with all Colt internals
    Magazines: PMags, Brownells Aluminum Mags, and Colt Al mags.
    The Ammo: Remington Green Box 55gr AND Federal black box 55gr xm193

    Any help would be appreciated.

    Thanks,
    Jason

    ps, on a side note, my new SCAR 16S ran without issues, and felt very soft shooting compared to a carbine length gas system AR.
    Last edited by RAM Engineer; 09-06-10 at 15:44.

  2. #2
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    Any pressure signs on the ejected cases?
    Pat

  3. #3
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    one had a slightly flattened primer, but the rest seemed normal.

  4. #4
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    This seems gas system related, or bolt. If it were consistently just short stroking, I'd look exclusively at the gas system (maybe try stock gas rings), and look for burnt powder indications along the gas tube.
    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

  5. #5
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    Gas problem is the likely issue. Check to ensure the gas tube is not installed 180 degrees off (as in the gas tube hole is not aligned with the gas port). While not common, the gas tube can clog, so check that air can pass through it. Replace if needed. Check that the gas key is tight and staked in place. Also not common, but it is possible to get debris longed in the gas key itself. You should be able to feed a pipe-cleaner through it. Trying a set of standard USGI gas rings is a good suggestions as well.

    Also hit the chamber with a chamber brush and clean/inspect to ensure there isn't debris in the lugs locking it up.

    Is this a new build that is not working, or did this just crop up after it had been working?

  6. #6
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    Sounds like a misaligned port/gas block.
    Did you fiddle with the gun after you got it back from ADCO, did they do any work on the gas system and if so did they test-fire it?
    What gas block/FSB is on it? If it is aftermarket, who put it on?
    I am very close to locking this thread due to our rule of: "Contact the manufacturer first", but it might not have anything to do with them.
    Last edited by Failure2Stop; 09-06-10 at 22:25.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  7. #7
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    The first complete upper I ever built years-ago was also a "single shot at it's first trip to the range. Turned out I installed the gastube upside-down!

    I would also suggest ditching the macfarland 1-piece rings, especially if you got them from Brownells. I ordered several sets a few months back from them and it turned out they were all undersized and the bolt would basically drop right out of the carrier due to the lack of friction.

  8. #8
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    If you have to mortar the gun to get the case out then it sounds like you have a chamber or headspace issue, especially with the slightly flattened primer.
    "The sword is more important than the shield, and skill is more important than either. The final weapon is the brain. All else is supplemental." John Steinbeck

  9. #9
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    Quote Originally Posted by Eric View Post
    Is this a new build that is not working, or did this just crop up after it had been working?
    New-ish upper. I bought it several years ago and had ADCO attach the FH, put it in the safe and forgot about it until this past week. This was the first time it was ever shot, other than whatever LMT and ADCO did with it.

    Quote Originally Posted by Failure2Stop View Post
    Did you fiddle with the gun after you got it back from ADCO, did they do any work on the gas system and if so did they test-fire it?
    What gas block/FSB is on it? If it is aftermarket, who put it on?
    I am very close to locking this thread due to our rule of: "Contact the manufacturer first", but it might not have anything to do with them.
    All I did after I got it back from ADCO was put a pair of carbine handguards on it and a Troy BUIS. All ADCO was hired to do was permanently attach the Vltor flash hider. I don't know if they test-fired it.

    It has a standard fixed front sight base on it. Factory LMT.

    My apologies for posting this without contacting LMT or ADCO first. I'm hesitant to contact either of them yet. Mostly due to the time span between when I bought the upper and shot it and the fact that ADCO worked on it prior to the first time it was fired.

    Based on what you guys are tellin me, I plan on the following course of action:
    1. Remove handguards & BCG.
    2. Inspect gas tube for proper orientation and clear gas flow.
    3. Inspect bolt carrier key for proper installation
    4. Thorough cleaning of chamber, barrel extension and locking lugs.
    5. replacement of gas McFarlane gas ring with OEM version.
    6. reassemble and test fire.

  10. #10
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    This is not a gas issue but a chamber or ammo issue. Good news is that you've got a good extractor cause it's hanging on for dear life.
    If you are lucky, it's the ammo. Seen it a bunch. Trained the 82nd ABN for three months a couple years back. They were force fed British 5.56. Horrible quality control compared to M855. Hundreds of 'Failure to Extracts'.
    Had a guy chamber a Jolly Rancher candy wrapper once and couldn't for the life of me figure out why so many failure to extract until deductive reasoning kicked in and I pulled the wrapper out w/ a a dental tool.

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