9mm case blown apart,Why?

The #1 community for Gun Owners of the Northeast

Member Benefits:

  • No ad networks!
  • Discuss all aspects of firearm ownership
  • Discuss anti-gun legislation
  • Buy, sell, and trade in the classified section
  • Chat with Local gun shops, ranges, trainers & other businesses
  • Discover free outdoor shooting areas
  • View up to date on firearm-related events
  • Share photos & video with other members
  • ...and so much more!
  • Bountied

    Ultimate Member
    Apr 6, 2012
    6,878
    Pasadena
    I have not yet seen a 9mm bolt for an AR with a free floating firing pin. The bolt moves much faster in a blow back system than in a normal AR gas system. A spring is the only thing keeping most from firing fully automatic as long as the trigger is held back.

    Free floated or not the pin was struck by the hammer with enough force to detonate the primer out of battery. I wasn't talking about a slam fire.
     

    GuitarmanNick

    Ultimate Member
    Jan 9, 2017
    2,221
    Laurel
    Free floated or not the pin was struck by the hammer with enough force to detonate the primer out of battery. I wasn't talking about a slam fire.

    I understand that this was not a slam fire.

    Since the bolt does not have any locking lugs on a blow back AR, and the firing pin and hammer are not blocked mechanically, it is not unheard of for a round to be fired out of battery. I was unaware of the possibility until after my Polymer80 lower was destroyed last weekend by an OOB detonation. The likely reason is that I increased the rate of fire on the last magazine. Too much inertia allows the bolt to bounce the round out of the chamber. It also happens too fast to be seen by the naked eye. If you pull the trigger while the round is OOB, it will detonate! In most cases, no damage is done and you can continue shooting after clearing the ruptured case from the chamber.

    Not all cases will rupture. I looked carefully at my brass from last week's outing and found evidence that about 10 rounds were fired while slightly out of battery before one finally ruptured and blew the P80 lower apart. They had very distinctive bulges and will go in the scrap bucket.

    A 9mm AR is nothing like the 9mm pistols we are accustomed to shooting.

    The OOB issue can be addressed by selecting a longer and/or heavier buffer. Based on the reading I have done this week, my 10.5" barrel should be paired with an 8 ounce extended buffer instead of the 5.3 ounce standard length that was installed at the time. The additional length and mass will slow the bolt's momentum enough to eliminate most of the bolt bounce.

    There is a lot of good information available on tuning your blow back AR to reduce the chances of an OOB detonation. Wish I had done a little more research before putting my build together. Ordered the new buffer and getting an aluminum lower for the rebuild. It is too much fun to throw in the towel!
     

    Bountied

    Ultimate Member
    Apr 6, 2012
    6,878
    Pasadena
    I understand that this was not a slam fire.

    Since the bolt does not have any locking lugs on a blow back AR, and the firing pin and hammer are not blocked mechanically, it is not unheard of for a round to be fired out of battery. I was unaware of the possibility until after my Polymer80 lower was destroyed last weekend by an OOB detonation. The likely reason is that I increased the rate of fire on the last magazine. Too much inertia allows the bolt to bounce the round out of the chamber. It also happens too fast to be seen by the naked eye. If you pull the trigger while the round is OOB, it will detonate! In most cases, no damage is done and you can continue shooting after clearing the ruptured case from the chamber.

    Not all cases will rupture. I looked carefully at my brass from last week's outing and found evidence that about 10 rounds were fired while slightly out of battery before one finally ruptured and blew the P80 lower apart. They had very distinctive bulges and will go in the scrap bucket.

    A 9mm AR is nothing like the 9mm pistols we are accustomed to shooting.

    The OOB issue can be addressed by selecting a longer and/or heavier buffer. Based on the reading I have done this week, my 10.5" barrel should be paired with an 8 ounce extended buffer instead of the 5.3 ounce standard length that was installed at the time. The additional length and mass will slow the bolt's momentum enough to eliminate most of the bolt bounce.

    There is a lot of good information available on tuning your blow back AR to reduce the chances of an OOB detonation. Wish I had done a little more research before putting my build together. Ordered the new buffer and getting an aluminum lower for the rebuild. It is too much fun to throw in the towel!

    Sounds like you have it all figured out. Good luck with your 9mm build.
     

    Users who are viewing this thread

    Latest posts

    Forum statistics

    Threads
    274,915
    Messages
    7,258,424
    Members
    33,348
    Latest member
    Eric_Hehl

    Latest threads

    Top Bottom