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Thread: Failure to fully cycle malfunction

  1. #1
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    Failure to fully cycle malfunction

    Ok, I realize that this might fall into the category of a short stroke, but would like to run this by you folks.

    I attended a two day carbine class in Sacramento this past weekend and brought out the following upper setup:

    * LMT 16" M4-type upper with ~1k through the tube prior to class.
    * Denny's super-duty AR15 BCG with ~400 through prior to class.
    * BCM Gunfighter Mod4 CH
    * Troy rear BUIS
    * DD Omega 7.0
    * Aimpoint T1 + LaRue LT660 tall mount

    The lower is a Bushmaster with a Magpul BAD and LMT SOPMOD kit with H-buffer.

    Ammunition run throughout the course:
    * 565 rounds of XM193 boxed
    * 160 rounds of PRVI M193

    Magazines run were all USGI with Magpul followers and Ranger Plates.

    Day1 was a flawless day and all rounds down range were with XM193. However, Day2 proved to be slightly different due to five occassions that had the same symptoms.

    - Action would cycle
    - Action would sort of lock to the rear on near empy magazines (7-8 rounds) but was not hung up by the bolt catch/release.

    The bcg would just seem to "hang up" nearly to the end of it's travel but just a bit shy of full rearward movement to allow a new round to be stripped. The PRVI was mixed in with the XM193 on Day2 as there were partially loaded mags leftover from Day1 that were filled the night before.

    Unfortunately, during the heat of the drills, I was unable to mark all the magazines used during the five malfunctions except for two of them. One of them looked great whereas the other one had a slightly bent feed lip on the right side. As a side note, I did realize that the malfs seemed to occur when an odd round was ejected and an even was about to the stripped.

    Upon cleaning and inspection of the rifle, the BCG looks to be in great shape. The bolt seems a bit loose in the bcg, but does not fall out or extend when tipped upside down.

    So, ammo or mag related?
    Last edited by JimT; 01-12-10 at 02:33.

  2. #2
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    Forgive my ignorance on AR-malfs as I've been pretty fortunate these past 10 years and have never experienced a failure to feed/extract or short stroke. I have on a few occassions met the double feed, but those were all remedied via Magpul followers.

    Well, I went back and reinspected my clean BCG and performed the following process:

    1) Remove firing pin retaining pin, firing pin and bolt cam pin.
    2) Extend the bolt in the bcg where it would be in the "unlocked" position.
    3) Stand the assembly on end, using the bolt as the support.

    I did this five times and on all five times the BCG collapsed on the bolt. The gas rings were removed and they looked pretty thin radially. I went ahead and replaced gas rings that have less than 1300 rounds on them with new items.

    I'm pretty sure I know the answer to this, but would that have caused my malfs?

    The inside of the bolt carrier does not look like a file and is rather smooth.
    Last edited by JimT; 01-12-10 at 03:12.

  3. #3
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    If your BCG collapsed while extended then yes you had bad rings and it should run good now. I would suggest (only based upon my experience) that you get some McFarland 1 piece rings. They work good and seem to last longer.

    Quote Originally Posted by JimT View Post
    Forgive my ignorance on AR-malfs as I've been pretty fortunate these past 10 years and have never experienced a failure to feed/extract or short stroke. I have on a few occassions met the double feed, but those were all remedied via Magpul followers.

    Well, I went back and reinspected my clean BCG and performed the following process:

    1) Remove firing pin retaining pin, firing pin and bolt cam pin.
    2) Extend the bolt in the bcg where it would be in the "unlocked" position.
    3) Stand the assembly on end, using the bolt as the support.

    I did this five times and on all five times the BCG collapsed on the bolt. The gas rings were removed and they looked pretty thin radially. I went ahead and replaced gas rings that have less than 1300 rounds on them with new items.

    I'm pretty sure I know the answer to this, but would that have caused my malfs?

    The inside of the bolt carrier does not look like a file and is rather smooth.



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  4. #4
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    Thanks for the input. I'll put in an order for a few of them.
    Definitely hitting the range this weekend to run some more drills and make sure that the weapon is wired tight.

  5. #5
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    I concur. It sounds like a gas ring wear problem.
    I suspect that the Prvi might have been the problametic rounds, though to my knowledge it isn't necessarily underpowered, but combine an LMT gas port, H buffer, and ring wear, and shoot non-NATO pressure ammo and cycling problems are bound to pop up.

    This is not to blame the Prvi, the gun should be able to run reliably with everything from Wolf to Mk262.

    Eta- in my experience, LMTs don't usually need H buffers, but should still work with them.
    Last edited by Failure2Stop; 01-12-10 at 04:45. Reason: H buffer
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  6. #6
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    Hey, thanks for your input.

    Are the LMT uppers known to be "undergassed?"

    Another tidbit of information:
    - spent cartridges were ejected in the 3-4 o'clock region.
    Last edited by JimT; 01-13-10 at 01:27.

  7. #7
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    Could it be the extractor claw spring? I mean I had the same problems with a non-gas piston AR. It got fixed when the spring was replaced..
    Perfect Training Makes Perfect

  8. #8
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    Quote Originally Posted by JimT View Post
    Hey, thanks for your input.

    Are the LMT uppers known to be "undergassed?"

    Another tidbit of information:
    - spent cartridges were ejected in the 3-4 o'clock region.
    They are correctly gassed.
    3-4 o'clock ejection pattern is right on the money.
    Also number the mags and keep an eye on them just in case.
    Last edited by Thomas M-4; 01-13-10 at 01:40.

  9. #9
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    Check that the hammer is not dragging on the carrier, I've seen it with a few two stage triggers.

    On an empty chamber pull the trigger and hold to the rear.
    Cycle the action and feel for drag.
    Release the trigger and if the drag goes away you have an issue with the trigger group.

  10. #10
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    Will do at lunch.

    The lower in question is my primary and easily has 10k on it with the same FCG. On the other hand, it is now running a newer upper assembly. It's original upper was another LMT unit that clocked 8k of flawless rounds.

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