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Thread: Primer punching?

  1. #1
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    Primer punching?

    I am the second owner of a CMMG AR-15 Model MOD4SA
    Over the past 100 rounds, I have had a number of problems mostly dealing with extraction, but also with blowing primers resulting in pieces that would get lodged in the Bolt assembly. The extraction issues were solved , but the primer blowing issues still remain despite replacing the firing pin with a new one from Bravo Company.

    At the range today, My instructor brought his AR and we compared spent brass. His brass appears normal (including using some of my ammunition (federal XM193) My brass showed a ridge around the depression made by the firing pin on all primers except those that were completely blown through (see attached photos)

    Any idea what is happening? Suggestions on repairs?

    Normal Spent brass:


    My brass:


    Worst of my brass today: light strike, heavy strike punchthrough and blowout


    My bolt:



  2. #2
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    Sounds like a tight chamber...
    How was the extraction issue resolved? What are the rest of the specs of the rifle?

  3. #3
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    Quote Originally Posted by Col_Crocs View Post
    Sounds like a tight chamber...
    How was the extraction issue resolved? What are the rest of the specs of the rifle?
    Replaced the extractor spring with an upgraded one from Bravo company.

    Gnu is a carbine length gas-operated MOD4SA with original BCG (minus upgraded extractor spring and replaced firing pin)

  4. #4
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    Not sure about cause of over pressure (I guessing that's what's going on here), but I thought you were suppose to replace firing pin every time you had a pierced primer as it would lead to even more.
    ^^ Read with southern accent !^^ and blame all grammatical errors on Alabama's public school system.
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  5. #5
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    Quote Originally Posted by lunchbox View Post
    Not sure about cause of over pressure (I guessing that's what's going on here), but I thought you were suppose to replace firing pin every time you had a pierced primer as it would lead to even more.
    Even after replacing the primer I still had this problem.

  6. #6
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    Has it been popping primers since day one of you acquiring it or just recently? I wouldn't put it past CMMG to have tight chambers but if this is recent, it might be something else causing the spike in pressure.

  7. #7
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    The first step is to give the chamber/barrel a thorough cleaning. You said you are the second owner so I wouldn't make any assumptions about the condition of the barrel/chamber. Fire some rounds after cleaning and see if the overpressure symptoms persist.

  8. #8
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    Quote Originally Posted by LewP View Post
    The first step is to give the chamber/barrel a thorough cleaning. You said you are the second owner so I wouldn't make any assumptions about the condition of the barrel/chamber. Fire some rounds after cleaning and see if the overpressure symptoms persist.
    I acquired the piece over 2 years ago. I noticed it was popping primers after about 200 rounds (which occurred over the course of several months as where I lived there weren't many rifle ranges nearby I could use at the time.)

    It hasn't left the safe much as each time I've been able to take it out to shoot, it had some kind of problem. I would bring it home, try to tinker/clean it and not take it out to the range for several months (just wasn't enjoyable to pay a range fee only to find out the gun doesn't work right) The gun has been cleaned several times including by my instructor who is also a S&W gunsmith. The primer punching problem still remains.

  9. #9
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    Was the chamber thoroughly cleaned, specifically?

  10. #10
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    Swap BCGs with a known good shooting rifle and see if the problem goes away or follows your BCG to the other rifle. This will enable you to find the problem or rule out your BCG and look at your chamber.
    'Evil Minds That Plot Destruction'

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