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Thread: Critical Failure - Seeking Advice

  1. #41
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    Glad to hear your carbine headspaces correctly.

    From the 1994 M4 MIL-SPEC ( http://biggerhammer.net/ar15/milspec...71186_(AR).pdf )

    3.4.1 Headspace. The headspace shall not be less than 1.4646 inches nor more than 1.4706 inches when measured to the .330 inch datum diameter on the first shoulder of the chamber.
    and how they check:

    3.4.4 Hiqh pressure resistance. Each barrel assembly and bolt shall withstand the firing of one Government standard M197, 5.56mm high pressure test cartridge conforming to MIL-C-46936. After proof firing, parts shall be free of cracks, seams and other injurious defects as evidenced by visual and magnetic particle inspection.
    4.7.1.1 Minimum gage. The bolt shall be moved rearward to the open position. The minimum headspace gage shall be inserted in the chamber and the bolt shall be returned to the battery position. Only finger pressure shall be applied to close the bolt. To be acceptable the bolt shall fully close.

    4.7.1.2 Maximum gage. The bolt shall then be retracted to the open position and the minimum headspace gage removed from the chamber. The maximum headspace gage shall be inserted in the chamber and the bolt returned to the battery position. Only finger pressure shall be used to close the bolt. To be acceptable the bolt shall not fully close.

    4.7.1.3 Dimensional inspection. If the above results are in dispute, the carbine shall disassembled and the 1.6206 + .0030 barrel dimension and the .15: - .003 bolt dimension shall be measured to determine compliance with the drawing requirements.
    The focus now goes back to barrel and/or your ammo.

  2. #42
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    I may be wrong but I think this doc is dated. As far as I know only the Field Gage is being used.

    Quote Originally Posted by sinister View Post
    Glad to hear your carbine headspaces correctly.


    From the 1994 M4 MIL-SPEC ( http://biggerhammer.net/ar15/milspec...71186_(AR).pdf )



    and how they check:





    The focus now goes back to barrel and/or your ammo.



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  3. #43
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    As of 2009, MIL-C-71186A keeps the same min-max headspace tests and adds another after the 50th 30-round mag cycle (1500 rounds after the proof round and grouping test fire):

    3.6.7.6 Endurance headspace measurement. Headspace for weapons being fired with 30 round magazines shall be measured and recorded at the beginning of the test and at the completion of the 50th cycle. After the 50th cycle, the headspace shall not be more than .0028 inch greater than the initial measurement and shall not exceed .0024 inch over maximum (1.4706 inches when measured to the 0.330 inches datum diameter on the first shoulder of the chamber).
    This accommodates any chrome that may have flaked or chipped off the chamber shoulder. Technically, anything longer than 1.4706 might allow a NO-GO gage to close. NO-GO is the safe LIMIT allowing the bolt to close. 1500 rounds is just burnishing the chrome from the throat and lands and the weapon should be nicely broken in.

    If a bolt closes on a FIELD gage it is unsafe to fire. Headspace will be too long, allowing the case to possibly be unsupported around the chamber chamfer and allowing a pressure rupture out of the extractor groove on the bolt -- like the OP's photos.

  4. #44
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    Quote Originally Posted by MK18Pilot View Post
    As long as the bolt is locked, the relative position of the carrier is irrelevant. This is an ammo related incident. It's clear from the pictures that case failed.
    EXACTLY. If the bolt rotated, it was in battery and that's all that matters.

    And the round could NOT have fired by the firing pin if the bolt was not in battery. Physicall impossible.
    "You people have too much time on your hands." - scottryan

  5. #45
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    The bolt could be locked, but if the chamfer or headspace is excessive (i.e., the distance from where the chamber walls end, the actual end of the barrel, and the bolt face/front of the bolt lugs begin could be wide open) the case walls aren't supported.

    I'd be leery of the ammo, but I don't know if Mossberg has ever been assigned a government contract for spare AR/M4 barrels with extensions, either.



  6. #46
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    As usual I am impressed by the knowledge base on this site!! Thanks guys.

  7. #47
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    I am not sure if this view will provide any additional insight to the case/ammo issue or not, but here is another photo I took tonight of the case showing another crack in the base of the case - along the primer pocket/hole - that wasn't visible in my previous photos.



    Gonna take the carbine out and shoot it tomorrow night and will report back with results.

  8. #48
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    I assume the PP 2012 is loaded to 5.56mm pressure.

    Is the Mossberg barrel throated for commercial .223 Remington or military 5.56mm?

    Second, is the barrel Melonited inside as well (I would assume so, the way they're treated)? A combination of short throat and extremely hard leade might raise your chamber pressure.

  9. #49
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    This failure is almost exactly like mine. The upper is a BCM and I know its wIthin spec. Since the same ammo has caused other issues in my case there is no doubt it was ammo related.



    Owner/Instructor at Semper Paratus Arms

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  10. #50
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    Quote Originally Posted by sinister View Post
    I assume the PP 2012 is loaded to 5.56mm pressure.

    Is the Mossberg barrel throated for commercial .223 Remington or military 5.56mm?

    Second, is the barrel Melonited inside as well (I would assume so, the way they're treated)? A combination of short throat and extremely hard leade might raise your chamber pressure.
    The barrel is marked as 5.56mm Nato, and yes melonite inside as well. I do not think I have any kind of gage to confirm this though.

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