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Thread: Previously reliable AR suddenly not cycling.

  1. #1
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    Previously reliable AR suddenly not cycling.

    So I went to the range this morning and my AR, which has been 100% reliable to date, abruptly quit cycling.
    I fired a few shots and then had one round get hung up half-fed into the chamber. Resumed shooting, another couple rounds and the bolt failed to lock open on the last round. Changed mags, chambered the first round and fired, ejection pattern seemed different and bolt failed to pick up the next round, skidding over the top of it instead, leaving a scrape on the brass.
    This pattern of chambering the first round, firing, different ejection pattern, and the bolt riding over the next round in the mag and not feeding continued until I gave up. Four different mags and two types of ammo were used all with the same results.
    The gun seemed to be short-cycling. I took it apart and noticed nothing amiss, as best I can tell the gas key is unobstructed.

    Rifle: BCM midlength upper on a lower with a rifle buffer and spring. Gun has been 100% until today when it has a complete meltdown.

    Ammo: Privi M193 and Radway Green SS109, gun has had no issues with either type, and several others, including Wolf, in the past.

    Magazines: two Magpul 20-rounders, a Magpul 10-rounder, and an Okay GI 30. None of these mags have ever had issues in this gun or in any others.

    Lube: Slip EWL30. My usual lubricant. Gun was amply lubricated.

    Ejection: normally this gun ejects at around 4 O'clock or so, I did notice that it seemed to be throwing them out at closer to 3 O'clock and not quite as far as usual. I didn't examine the brass as it landed in the snow but in hindsight I wish I'd dug some of it up.

    Environmental factors: approximately 30 degrees. I add this because I'm wondering if perhaps the thicker Slip EWL may have been gumming up the gun somehow in the cold weather. EWL is my regular lube but this is my first time shooting this gun in weather this cold. Although a week ago I shot the gun in only slightly warmer 33 degree weather, with the same ammo and mags, without issue.

    Any ideas?

  2. #2
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    I had a rifle do this. The gas key was loose.

  3. #3
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    Sounds like it's maybe short stroking. Does the BCG move freely and smoothly when you hand cycle it? If the gas key isn't loose are your gas rings in good order? Does the gas tube look good or is the raised part on the outlet wore off?

  4. #4
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    I checked the gas key, it's a BCM and is staked well, and it *seemed* alright in my initial examination but perhaps I need to look at it more closely.

  5. #5
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    Quote Originally Posted by TomMcC View Post
    Sounds like it's maybe short stroking. Does the BCG move freely and smoothly when you hand cycle it? If the gas key isn't loose are your gas rings in good order? Does the gas tube look good or is the raised part on the outlet wore off?
    Yeah everything seems alright. The gas rings were changed out maybe a couple hundred rounds ago and seem alright. Gas tube looks fine too as far as I can tell but I might have to go back over the gun's components with a fine toothed comb. It's just odd to me that it went from previously perfect to total dumpster fire.
    Last edited by Circle_10; 12-10-17 at 10:28.

  6. #6
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    Is the upper fully stock and unmodified? Could the gas block have gotten loose?

  7. #7
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    Quote Originally Posted by TF82 View Post
    Is the upper fully stock and unmodified? Could the gas block have gotten loose?
    The only alterations I've made to the upper are things like the handguards and the flash hider, I haven't touched the gas system at all. The FSB doesn't seem to be loose

    Double checked the gas key screws, they seem fine.

    Took a bunch of pics. Does it look like I might have a gas leak somewhere?







    Last edited by Circle_10; 12-10-17 at 12:12.

  8. #8
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    Gas tube wear? It is hard to say from the last picture, but it looks like the mushroom at the end could have worn down a bit. That could be a cause for the symptoms you are experiencing. Slightly misaligned gas tube is often missed in regular cleaning and inspection, and it causes accelerated wear to the gas tube mushroom.

  9. #9
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    Maybe it's just me, but I see no roll pin in your FSB. If the roll pin somehow fell out, your gas tube might have moved back a tad causing insufficient gas to cycle.

  10. #10
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    There is a roll pin present, it just doesn't show up well in the pics.

    Quote Originally Posted by Titan74 View Post
    Gas tube wear? It is hard to say from the last picture, but it looks like the mushroom at the end could have worn down a bit. That could be a cause for the symptoms you are experiencing. Slightly misaligned gas tube is often missed in regular cleaning and inspection, and it causes accelerated wear to the gas tube mushroom.
    It doesn't appear particularly worn when compared to a couple spare BCM tubes I have on hand, a new one and one that had seen light use. Unfortunately those two are both carbine-length ones.

    My best best seems like it might be to preemptively order a new gas tube and replace the one in the gun and clean the BCG etc off and perhaps try the thinner Slip 2000 vs the thicker EWL30.
    This has been a really good gun up until this morning and I'd like to get back up and running again ASAP.

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