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Thread: Blown primers and other malfunction troubleshooting

  1. #1
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    Blown primers and other malfunction troubleshooting

    I had some issues yesterday that I don’t understand. Would appreciate any help in getting it sorted out. I’m real bad about diagnosing malfunctions in the moment since I usually just go into immediate action but some of what I recall seems very odd.

    Barrel: 12.5 Criterion Core
    BCG: Geissele REBCG
    Ammo: 56gr 5.56 MEN
    Mags: multiple Gen M3 PMAGs and USGI
    Buffer: A5 w/ standard rifle spring
    Can: KAC 5.56 QDC
    Upper’s total round count: 399

    The upper was pretty new with only 204 non-suppressed rounds on it before the class. Those rounds were a mixture of cheap brass cases .223 so this was my first time with the MEN in this gun. The malfunctions started towards the end of the day after about 120 rounds. I kept the can on until I started having issues.

    First malfunctions were failures to feed from three different mags. It didn’t look super dirty but I initially assumed that was the issue since I had a can so I just added lube and kept going. Then I had a mixture of blown primers and rounds stuck in the chamber. The first primer lodged itself in between a lug in the barrel extension and the case’s rim, so that the round could not be fully chambered. I had to use a knife to pry it out. I then removed the suppressor and within the next mag or two, had another blown primer. I don’t remember the exact reason but the gun just felt weird when cycling so I stopped shooting and looked. The primer was in between the bolt and bolt catch, stopping the BCG basically at the rear of the cycle. This primer was dislodged by pulling the CH back.

    The last was an unfired round in the chamber. I don’t remember why I stopped shooting or what even tipped me off about it, but for some reason, the bolt was locked back (I very well may have dropped the mag and locked it to the rear). I do know that I glanced into the chamber and was very surprised to see a round fully seated with no mark on the primer. I let the bolt go (don’t remember if I did this gently or just dropped the bolt catch), pretty sure I pulled the CH with nothing and then dropped the CH and again pulled it and this time the round was extracted.

    I know blown primers can be a sign of over gassing but I had the barrel gauged before JT @ RAW assembled it and think it was .068 which seems reasonable. I know MEN is supposed to be hot and based on estimated velocity yesterday, it was, but am surprised by the mix of blown primers and failures to feed.

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    Last edited by Wake27; 01-17-23 at 06:35.
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  2. #2
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    We shot a little bit of that MEN with no pressure signs. I'd clean the hell out of the gun and re-assess.
    "What would a $2,000 Geissele Super Duty do that a $500 PSA door buster on Black Friday couldn't do?" - Stopsign32v

  3. #3
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    Expect a suppressed carbine-gas 12.5 to dump a lot of fouling into the upper.

    A suppressed Colt 633 Commando is filthy after 150 rounds of 5.56 (military or civilian).

    I would suspect the Criterion (typically with a tighter Wylde chamber for precision, vice a big, fat, .mil 5.56 chamber) is going to have more "Stiction" and be more sensitive to fouling. A carbine-length gas tube is also going to start un-locking while pressurized with case still obturated against the chamber.

    Take a look at Criterion's video at the 2:11 mark and see how much gas is coming back through the chamber on a suppressed rifle (then multiply by five 30-round magazines for 150 rounds. You can see air distortion as high-speed gas bleeds from the vent holes, then the obvious smoke):

    https://youtu.be/8axSRCz_tMs?t=131

    You walk a tight balance between precision and functioning while fouled with a suppressed Commando. A few remedies are a chrome-lined GI chamber, mid-length gas, a flow-through suppressor, and piston-operation. An XH heavy buffer and maybe an LMT improved carrier might help with cycle timing but offer nothing for handling pressure.

    Your cases are holding a lot of pressure and the primers are letting go as peak system pressure is trying to blow them out of the pockets before the bullet un-corks. I can only assume MEN crimps their primer pockets, otherwise you'd lose a bunch more. Check your bolt face for annular gas leakage around the primer pocket crimp. MEN is typically very good ammo.

    Good luck.

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    Ahh.. the good old Wylde Chamber.

    I hate to admit it here, but I flat out HATE shooting suppressed ARs. I'm MUCH happier putting on ears and letting my AR breath naturally. With the can everything is dirty. Your brass, your BCG, you air in your face.
    "What would a $2,000 Geissele Super Duty do that a $500 PSA door buster on Black Friday couldn't do?" - Stopsign32v

  5. #5
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    Quote Originally Posted by sinister View Post
    Expect a suppressed carbine-gas 12.5 to dump a lot of fouling into the upper.

    A suppressed Colt 633 Commando is filthy after 150 rounds of 5.56 (military or civilian).

    I would suspect the Criterion (typically with a tighter Wylde chamber for precision, vice a big, fat, .mil 5.56 chamber) is going to have more "Stiction" and be more sensitive to fouling. A carbine-length gas tube is also going to start un-locking while pressurized with case still obturated against the chamber.

    Take a look at Criterion's video at the 2:11 mark and see how much gas is coming back through the chamber (then multiply by five 30-round magazines for 150 rounds. You can see air distortion as high-speed gas bleeds from the vent holes, then the obvious smoke):

    https://youtu.be/8axSRCz_tMs?t=131

    You walk a tight balance between precision and functioning while fouled with a Commando. A few remedies are a chrome-lined GI chamber, mid-length gas, a flow-through suppressor, and piston-operation.

    Your cases are holding a lot of pressure and the primers are letting go as peak system pressure is trying to blow them out of the pockets before the bullet un-corks. I can only assume MEN crimps their primer pockets, otherwise you'd lose a bunch more. Check your bolt face for annular gas leakage around the primer pocket crimp. MEN is typically very good ammo.

    Good luck.
    Wylde chambers (design)are actually larger than a 5.56 chamber, the freebore where the bullet rests is a little shorter.

  6. #6
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    Blown primers and other malfunction troubleshooting





    Quick shots of the BCG. It’s not clean but that should still be functional right? There was a lot of gas blowback in their promo vid which is interesting. I didn’t notice any the entire time except for the first 2-3 rounds right after I dumped lube on it.

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    Last edited by Wake27; 01-17-23 at 18:45.
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  7. #7
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    I'm leaning towards the chamber. I don't remember MEN 5.56 being super hot, but those wylde chambers can be an ass ache for sure.
    "What would a $2,000 Geissele Super Duty do that a $500 PSA door buster on Black Friday couldn't do?" - Stopsign32v

  8. #8
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    We were estimating that it was moving somewhere around 2800 FPS but didn’t actually chrono it. I don’t know anything about loading but I do know that the MEN doesn’t have crimped primers. Could that be contributing?


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  9. #9
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    Quote Originally Posted by Wake27 View Post
    We were estimating that it was moving somewhere around 2800 FPS but didn’t actually chrono it. I don’t know anything about loading but I do know that the MEN doesn’t have crimped primers. Could that be contributing?


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    100% Blown Primers are an AMMO issue and have nothing to do with the gun.

    If you change up ammo I bet you will have no more issues.

    Real 556 should have crimped primers, sounds weird yours are not.
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  10. #10
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    Quote Originally Posted by turnburglar View Post
    100% Blown Primers are an AMMO issue and have nothing to do with the gun.

    If you change up ammo I bet you will have no more issues.

    Real 556 should have crimped primers, sounds weird yours are not.
    I knew it was maybe the most common issue but didn’t think it was 100% the fault of the ammo.


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