Out of battery firing

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  • ScouT6a

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    Ok, so I am running a blow back operated 9mm at around 800 rpm. Out of 150 rounds fired, 4 of them resulted in bulged cases, at the base. One, very dramatic. Looks like you tried to blow it up like a balloon.

    So my question is, how would you determine if the gun is firing early (before the round is completely in the chamber) or if the bolt is extracting the case too quickly after firing?

    Next, thoughts as to if it's a buffer weight issue or gas pressure issue or both.
    Rounds are 124 grain hard cast over Hi Skor 700x powder.

    Thoughts, ideas??
     

    Thor

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    Could be anywhere
    Soft primers and a free floating firing pin? I wouldn't expect the pin to be accelerating forward though until it went into battery unless there was a slight drag during chambering. Other than that maybe a dirty pin channel not allowing full retraction so that the pin is exposed as it's chambering a round. Good luck.
     

    ScouT6a

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    Firing pin spring is new, less than 500 rounds. I will break the bolt down tonight and check the cleanliness.
    Thought about adding a couple of grams of weight at a time to the buffer (temporarily) and see if that changed anything.
     

    ScouT6a

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    20180406_082341_zpsyfotxf5o.jpg
    [/IMG]
     

    bwframe

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    Wow. Holy crap, that's scary. :eek:

    I've shot a fair amount of 9mm through my self-built AR and seen nothing like that.
     

    2A_Tom

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    I would guess hesitation at the chamber mouth. The rim of the casing hitting and hesitating at the mouth of the chamber. Polishing the entrance to the chamber may fix the problem.

    There is a possibility that adding weight to the buffer could exacerbate the problem.
     

    Leadeye

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    I've fired many thousands of 9mms through a MAC 10, fixed firing pin, fast rate, blow back operation and never have seen anything like that. I would guess firing just out of battery like blue falcon is suggesting with the case being slowed before fully clambering.
     

    praff

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    Edited...I just realized this was full auto we are talking about. :ugh:


    Factory upper?? or one that you assembled?? I would still probably check headspace.

    Firing pin could be getting stuck in breach face and not completely retracting.

    Also, could have been a few rounds that just didnt chamber well due to not being sized completely or previosly bulged.

    Something that just started happening on an existing upper/lower???

    I suppose timing could be off a bit with sear trip and bolt as well. Is it an M16 style lower or a drop in sear type??
     
    Last edited:

    rvb

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    bolt bounce can be a problem too. make sure buffer has dead weights that are free to move. that is where a too-light spring could also lead to this result. edit: you likely wouldn't see effects of bolt bounce in semi-auto. do you only see this problem in full auto?

    Assume you're using a heavier pistol buffer vs regular carbine buffer?

    I would look closely at belling/crimp and make sure bullet diameter is .356 max unless you've slugged your barrel. 9mm being slightly tapered, a wide mouth could lead to it only mostly seating. I would also check seat depth consistency / bullet shape... anything that could keep a round from feeding all the way. plunk test 100 rounds into your barrel and see if the problem persists...

    -rvb
     
    Last edited:

    praff

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    bolt bounce can be a problem too. make sure buffer has dead weights that are free to move. that is where a too-light spring could also lead to this result.

    Assume you're using a heavier pistol buffer vs regular carbine buffer?

    -rvb


    My post dealer sample ran much better once I went to a 308 spring and a heavier as well as longer buffer (most factory 9mm buffers are longer than a carbine). I was running a standard carbine buffer with a spacer to limit rearward travel. However, I never saw an out of battery condition like that.
     
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