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Interesting AR-308 Failure

Discussion in 'Gunsmithing and Repairs' started by carnaby, Feb 7, 2013.

  1. carnaby

    carnaby Well-Known Member

    Had a 308 AR fail the other day and I am trying to understand what happened. Here's what I know:

    1. barrel rotated 20 to 30 degrees in the receiver, index pin snapped at some point allowing the rotation. Barrel nut was torqued to approx 50 ft-lb.

    2. Carrier moved back with bolt about half way out of the carrier from lockup.

    3. Case is ruptured but don't know when the rupture happened, if it was cause or effect. Case was partially out of the chamber when I got the rifle back.

    4. Pressure from the ruptured case blew out the bottom of the magazine.

    5. Carrier is cracked where it was forced against the cam pin. Bolt has marks behind the lugs.

    6. The ejector is protruding about 1/16" pas the bolt face. Removing the ejector showed that the ejector retaining pin is mangled to some degree.

    7. Bolt is JP enhanced, carrier is Young Mfg, barrel is Bison Armory with BAT Machine extension, receiver is Mega MA-TEN.

    Here's the receiver with the gouge visible:


    And the other parts. A new JP bolt is shown for comparison:


    Ammo was Federal Gold Match .308 Win 175 grain SMK. Brass is pretty banged up from being wacked with a hammer to get it out of the chamber. The case and bolt were locked together pretty well and all jammed up in the receiver. Some of the gouge in the receiver may be due to the steps that were taken to get the BCG and case out.

    Any thoughts?
  2. MarshallDodge

    MarshallDodge Well-Known Member

    Has this rifle ever been head spaced?

    Bullet setback?
  3. ApacheCoTodd

    ApacheCoTodd Well-Known Member

    Was this the first round after assembly?
    I'd like to see the index notch on the receiver.
    Which direction was the barrel rotated? With the tightening torque?
    I'm assuming without seeing it in hand that the rotation was sufficient to allow the firearm to cycle almost in a "blow-back" mode.
    That firing pin retaining pin is a big heads-up as to the jolt that the firing pin received.
  4. taliv

    taliv Moderator


    never heard of that pin snapping before.

    my guess is bbl was rotated ccw
  5. Trent

    Trent Resident Wiseguy

    If the barrel rotated, wouldn't it'd cut off the flow of gas through the gas port? The action shouldn't have cycled.

    What was the lead-up to this point like? Was the rifle shooting fine? And then BOOM?

    If the gas flow is cut off, it's essentially a bolt action. That IN ITSELF is not an unsafe condition - the British shooters fire non-gas-system AR's all the time, manually cycling them with the charging handle.

    Even if the index pin broke, and the barrel rotated, the WORST that should happen would be the gas supply is cut off. The barrel itself will still be retained in the appropriate spot as the barrel extension ring is pinched between the nut and the receiver. But, a rotated barrel wouldn't even chamber, as the locking lugs wouldn't line up with the bolt. The rifle wouldn't chamber or fire, it would be so far out of battery as to not allow the hammer to strike the firing pin.

    (Or WOULD it???? I've never actually TESTED that.)
  6. Trent

    Trent Resident Wiseguy

    Upon looking at that brass, and where it ruptured, it appears that the bolt was NOT engaged inside the barrel extension. That's a heck of a lot of case rupture. There's no way that could have been fully in the chamber.

    So my theory:

    Your pin snapped.

    Barrel rotated.

    SOMEHOW a round managed to get in the chamber with a misaligned feed ramp (THAT had to have been sheer luck)

    The bolt closed in FRONT of the locking lugs.

    When the trigger was pulled, the hammer must have been able to transfer enough force to pop the primer, detonating the case out of battery. (Pure luck that the primer detonated instead of just shoving the casing further in to the chamber)

    That's a pretty frigging serious flaw, starting with that barrel, and ending with the rifle capable of firing substantially out of battery.
  7. Trent

    Trent Resident Wiseguy

    I thought of a way to test this theory without mangling a barrel to do it.

    If the bolt closed on the FRONT of the lugs, and managed to fire out of battery, it should be possible to insert a thin piece of flat round metal (piece of a soda can? thin sheet steel?) over the locking lugs. Allow the bolt to close on it (outside of the locking lugs), and see if the gun fires out of battery. Any potential primer strike should be visible on the thin metal.

    I've never even given thought to that specific condition before, and I'm not sure if it's possible. But given the look of his brass, that barrel pin being sheared, and the resulting damage, it's the only idea I can come up with.
  8. Trent

    Trent Resident Wiseguy

    OOOH another idea. And this one is better.

    If the barrel was rotated slightly, AS the bolt started closing OUTSIDE of the locking lugs, the ROTATION of the bolt face would PROBABLY allow the lugs to LINE UP with the cockeyed barrel extension and (mostly) finish chambering the round.

    NOW you have a completely UNSUPPORTED bolt that DOES clear enough to allow the hammer to strike the firing pin.

    Pull the trigger, BANG, the whole bolt assembly immediately starts sliding BACK out of the cockeyed barrel extension and the case ruptures as it's being forced out under it's own recoil. Gas vents, blows out you mag, damages pieces of the bolt.

    Game over.

    I think THAT is the failure scenario you encountered.
  9. ApacheCoTodd

    ApacheCoTodd Well-Known Member

    Kinda covered the "blow back".

    The bolt went to the rear upon firing till the cam pin in its rearward position 9instead of forward as it should have been) struck the rear of the receiver cut-out. Enough travel for the rupture required to take place in the barrel extension rather than full in the chamber.

    My guess is still that this went to battery, fired and not having the bolt locked due to the rotation of the barrel - the combustion was finished out of battery.

    There's your receiver mar, bent firing pin retaining pin, ruptured case and cracked carrier.
    No gas needed for this failed cycling as the bolt was not properly locked into battery being off no doubt nearly a complete lug - something along the lines of near the 45 degrees to the next indexing. It shouldn't have to be the full 45 due to beveled edges which should be present on the lugs of the bolt and or the receiver extension.
  10. carnaby

    carnaby Well-Known Member

    The barrel definitely wasn't rotated 45 degrees, more like 20. The gas block was still located properly over the gas port, and the gas tube was still in the chamber. Unfortunately I don't know the sequence of events as I wasn't there.

    Not sure the total round count, but this was NOT the first round through the gun. Headspace was checked before, and then after the failure and is good.

    The round before this round was a light primer strike, didn't go off. Rifle may or may not have been in battery, nobody noticed, they just ejected the round and then fired the round that led to the KB.

    It's possible that the round was overpressure, or that the index pin had already snapped, or both, and upon firing the hot round the barrel rotated just after ignition allowing the carrier to move to the rear to early while pressure in the chamber was still high, which would then rupture the case and lead to what happened.

    In this case torque on the barrel would be opposite to bullet rotation as that's the only source of torque just after ignition an initial bullet travel. This at least agrees with what I saw... right hand threads, barrel rotate approximately 20 or so degrees left. Equal opposite reaction and all.

    The bolt doesn't look any worse for wear but the carrier is definitely done.

    I'm not sure that the barrel could have been rotated before the KB, seems less likely, but I'm sure it's possible. I'm most perplexed by the snapping of the index pin and the rotation of the barrel. Any thoughts? Overpressure round? Bad index pin? At 55 ft-lb of torque would the barrel rotate without the index pin under otherwise normal circumstances?

    Upon closer inspection of the broken index pin in the barrel extension, its face appears to have an internal void, such that it would have been more prone to snapping.

    In any case, where did the torque come from that lead to the snapping of the index pin and the subsequent barrel rotation? The muzzle device was installed with shims, not a crush washer, and install torque was mild, maybe 20 ft-lb. The pin should have been fine after install and the barrel was straight.
  11. carnaby

    carnaby Well-Known Member

    The index pin appears to have snapped without much prevailing torque. As evidence the slot for the index pin in the receiver is only lightly deformed from pressure of the pin against the side of the slot. Compared with what you see when someone has snapped the index pin from over-torquing a flash hider this is minimal.
  12. ApacheCoTodd

    ApacheCoTodd Well-Known Member

    So, you're saying the barrel was still properly indexed in relation to the gasport/block/tube indexing but the extension turned on the barrel?

    ""I don't know the sequence of events as I wasn't there.""

    Seems you may not be getting all the information by omission, guilt or ignorance of the actual particulars by the owner/shooter.
  13. carnaby

    carnaby Well-Known Member

    Conducted a little experiment. Re-installed the barrel without the index pin and torqued the barrel nut to 50 ft-lb. Then torqued the barrel until it rotated in the receiver, which required 55 to 60 ft-lb. So some sort of large impulse was applied to the barrel to get it to rotate.
  14. carnaby

    carnaby Well-Known Member

    I really don't think the rifle fired out of battery as I doubt the bullet would have made it out of the barrel given the lack of pressure in the bore. The barrel is clear so I'm fairly confident the action was locked up when the trigger was pulled.
  15. carnaby

    carnaby Well-Known Member

    No, the barrel and extension are still in correct relation, the extension didn't come loose. The extension and barrel rotated in the receiver.

    That is possible.
  16. Trent

    Trent Resident Wiseguy

    The pinned gas tube would have applied spring tension to prevent it from rotating TOO far out of spec.

    Makes sense, the barrel extension is still in the same place it was before.

    Which really make me think it went down like I said in the last post I made. ApacheTodd's post got me thinking more about what would happen if the barrel were allowed to rotate freely. Due to the torque pressure of the nut, and the later spring "tension" provided by the gas tube.

    Imagine that on the LAST round that was fired, PRIOR to the light primer strike, the rearward motion of the bolt starts. The lug is already sheared (has been weakening over time due to the void.)

    The gas tube is still lined up, gas impulse arrives at the key. The bolt carrier begins moving back, rotating the bolt; but there's pressure against the lugs, and the barrel STARTS to rotate - just a little - with it.

    The round ejects.

    Bolt carrier starts moving forward.

    Picks up a round.

    But this time the lugs are out of position. When the front of the bolt face reaches the lugs, they don't line up; so the bolt face starts to CLOSE on the bolt; rotating due to the slant cut in the carrier.

    Suddenly SNAP they line up and the momentum carries the bolt forward a bit.

    But it lost enough momentum and it's badly out of battery.

    Shooter fires - no boom! It's not far enough forward at the time of the hammer strike to transfer energy through the firing pin. However the bolt carrier budges forward a touch more.

    Shooter yanks on the charging handle. Bolt carrier moves back - but the bolt lugs are in a freely rotating barrel. Barrel shifts over MORE.

    Next round is chambered. Bolt carrier and bolt goes forward, bolt face impacts lugs (again!). Bolt begins twisting - doing it's job when the bolt gets rearward pressure. Lines up with the lugs, slips in to the chamber.

    At this point the barrel is rotated nearly one full lug from it's original position (say, 20-30 degrees).

    The bolt carrier and bolt are far enough forward this time to allow the hammer to transfer energy properly to the firing pin.


    Cartridge goes off. IMMEDIATELY starts to get thrust OUT of the chamber, NO RESISTANCE!

    Case ruptures after 1/2 inch of rearward travel. Crap starts blowing apart.

    The bolt itself receives such a nasty hit that things inside start bending and twisting. The retaining pin is gouging in to the receiver and ultimately stops the rearward travel of the bolt.

    (Good thing too, otherwise it might have been a nastier failure).

    I seriously doubt it was caused by ammo. You have a failed part. And a solid theory now of how the failure occurred.

    It wasn't the bullet's twist that did it, it was the way the bolt twists within the bolt carrier. When the bolt starts moving backwards, there's still significant backpressure between the lugs on the bolt and the lugs on the receiver extension. The pressure drops quickly, as the bolt carrier diagonal cutout causes the bolt retaining pin to rotate the bolt and clear the lugs.

    That bolt took one HELL of a hard hit. I'd be wary about using it again. The LUGS would be fine. But man, the shearing force it was under around the bolt retaining pin would have been frigging incredible. I've had bolts snap in half there before, it's a weak spot. (But not a bad failure, generally doesn't hurt anything else when it happens).

    The barrel was almost certainly rotating for at least one, and perhaps many more shots prior to that kaboom.

    Then, it was just a matter of time before the part failed. This stuff happens. We don't x-ray our gun parts, generally, to find hidden flaws. (I've seen old black powder cannoneers x-ray their cannons before, but those are odd folk.)

    The bolt itself.
  17. SilentScream

    SilentScream Well-Known Member

    I think the culprit may have been an overpressure round. This in turn could have set the sequence of events into motion.

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