Fired case from out of battery AR-15?

[EricBu]

I sat here with a BA and an upper playing with it, trying to figure out some way to make that happen. I ain't smart enough to come up with anything. Any scenario I imagine, ends with the case body being unsupported and blowing out. It's more likely that somebody else was fireforming brass with light charges and oatmeal and just happened to be using the same brass as the OP and dropped one, than this was caused by an OOB detonation in an AR.

 

[AK Hunter]

It looks like it was fired out of a 350 Legend.

 

[denton]

Here are the pictures of a 5.56 cartridge fired from an M16, with a case head failure.

At the time I was doing an emergency problem solving consultation at LCAAP on the M855, which was having an unfortunate tendency to break up after leaving the muzzle. In the middle of that emergency, the plant operator had two large trucks loaded with M855 that the Army was not allowing ship because of the above failure. Odd thing was, although the staff there was quite competent, almost none were avid shooters. So I looked at the pictures for a while and then told them this was a firearm failure, not a cartridge failure. They were greatly relieved, and the Army allowed the trucks to roll.

Here is what I wrote about it in my report:

The evidence is that this cartridge was fired without the bolt being fully locked up. Otherwise, it would be impossible to form the “magnum belt” now seen at the rear of the cartridge. If the bolt were locked, the chamber would not have allowed the belt to form.


Either the firing pin fell prematurely, discharging the cartridge before lock up, or the bolt failed to lock completely and the cartridge backed part way out in the process of discharging. That is something that the ATK gunsmiths know much more about than I do. But of this I am sure: The bolt was not locked up when this cartridge discharged.

So back then, I was acknowledging that it could be a premature primer strike or premature bolt unlock. I've just assumed since then that a piece of grit had jammed the firing pin forward, and the momentum of the bolt carrier had supplied enough force to fire the primer. But it's clear that it could also be premature bolt unlock.

In the midst of both of these problems, I took time to make the acquaintance of the gunsmith staff. They had a wealth of useful information, and nobody was paying attention to what they had to say.

 

[EricBu]

It looks like it was fired out of a 350 Legend.

Or 300 BLK

 

[243winxb]

High pressure.

PSA PA-15-2 Looks like the action was locked. If i remember correctly, the round was fired into a bullet, lodged in the barrel from a squib?

 

[denton]

High pressure.

PSA PA-15-2 Looks like the action was locked. If i remember correctly, the round was fired into a bullet, lodged in the barrel from a squib?

OUCH!

 

[Varminterror]

OOBD’s in AR’s are NOT caused by the floating firing pin becoming stuck forward.

Remember, AR-15 FP protrusion spec is only 28-36thou, and the bolt travel during the locking cycle is over 250thou, meaning when the bolt is unlocked, the firing pin cannot even reach the bolt face by nearly 10x its protrusion…

So OOBD’s are caused predominantly by high set primers, with a much smaller subset occurring by broken FP’s, debris like stacked pierced-primer fragments in the pin bore, etc.

I suppose a broken cam pin could sufficiently bind the FP to cause a slamfire but also allow the bolt into battery sufficiently far to fire, but NOT lock - but again, we typically would expect the case to rupture as the BCG freely moves backwards like a blowback action, and of course, the broken cam pin would be immediately obvious upon inspection of the firearm after this event. Equally, a broken and seized FP could cause an unlocked detonation, slam fire, however, again we’d expect a ruptured case and the broken firing pin would be obvious upon post-event inspection.

 

[Shrinkmd]

It's more likely that somebody else was fireforming brass with light charges and oatmeal and just happened to be using the same brass as the OP and dropped one, than this was caused by an OOB detonation in an AR.

Trust me, it’s mine. 100 cases went to the range and 100 came home…. How many people are shooting reloaded Prvi Partisan cases from M193 they bought from Wideners back in the day…when it was $300 a case…

 

[GW Staar]

The fact that the primer was bulging a little and the primer pin dent pushed back out, makes me think too much powder made it into that one round. Not an expert, but if that happened, is there a scenario where even the excess pressure could not accelerate the bullet out the barrel fast enough to allow all the pressure wave to go forward......yet at the same time, the bolt return was not fast enough to pull ALL of the case ALL the way out of the chamber before that residual pressure wave fire-formed the neck to the base diameter?

Seems to me Shrinkmd was lucky the bolt WAS that slow, or the blast might have hit his face. Make any sense? That may have been the luckiest shot you ever made.....or God was watching over you!

Or could it have been the first shot of the day and something partially blocked the end of the barrel?

 

[Varminterror]

Seems to me Shrinkmd was lucky the bolt WAS that slow, or the blast might have hit his face.

In general, the AR-15 is well designed, such nothing really is ejected towards the shooter’s face during an OOBD, as I described on pg1:

If they truly fire out of battery, sufficiently unsupported, it’s really uneventful. They blow the bottom out of the mag, brass shrapnel out of the port, bending the top rounds in the mag and ruining the box, so it’s certainly exciting, but it’s overall underwhelming for what we might expect to happen.

 

[Charlie98]

Not an expert, but if that happened, is there a scenario where even the excess pressure could not accelerate the bullet out the barrel fast enough to allow all the pressure wave to go forward......yet at the same time, the bolt return was not fast enough to pull ALL of the case ALL the way out of the chamber before that residual pressure wave fire-formed the neck to the base diameter?

I was trying to think of an instance similar to that... where there was somehow two pressure waves... the first enough to drive the bullet down the barrel, past the gas port, then another secondary wave hit the port after the bolt started to unlock... a clump of powder ignited late, some sort of weird pressure anomaly or partial hang-fire? Dunno... just spitballing here.

I would think if there was some sort of protrusion on the bolt face... a chunk of something, piece of old primer, etc... that the impression of that would have been left on the primer cup.

 

[someguy2800]

I think the clue about the gun doubling might be the key. I've seen video of the carrier bouncing on closing in an AR far enough where it was nearly coming out of battery. My guess is there was a hammer follow that ignited the primer and the carrier was bouncing back as the pressure built up, so the bolt was all the way forward but was in the process of unlocking itself as the pressure took off. Then the gas got to the port and started extracting the case while it was still burning because the carrier basically had a head start. Normally the carrier moves a short distance before the cam pin reaches the ramp to start unlocking so there is a built in delay so the pressure can drop, but if the carrier was already bouncing open it would have a head start and would unlock early.

 

[Charlie98]

I've seen video of the carrier bouncing on closing in an AR

I watched a slo-mo on the M14 action... and I was surprised, you could clearly see the bolt bounce. It was more obvious there because you can see the bolt cam out of battery, unlike the AR, which is buried in the receiver. Knock me over with a feather...

 

[Shrinkmd]

I am looking carefully at the bolt. What are those two little erosions near the firing pin hole? They don’t seem raised. Do you think this is damage related to what occurred? Do I need a new bolt?

 

[BreechFace]

If you look at your brass head stamp under the second P on PPU it looks like there could be gas escape at that point on the primer.

What you show on your bolt could be how it came as well. It does line up on the edge of the primer mark on the bolt though.

Looks like it could be some flame cutting in your scenario where your OOB scenario allowed the case to push forward.

To me @someguy2800’s idea of bolt bounce makes a lot of sense on how this could possibly occur in the Stoner design. It would be good for you to measure and count the rings on your buffer spring and weigh your buffer and BCG.

Something is out of spec.

 

[Blue68f100]

What are those two little erosions near the firing pin hole?

They look like flame cutting from blown primers. What happens when you overload rounds and start blowing primers.

What does the locking lugs look like that contacts the barrel extension nut?

 

[BreechFace]

What does the locking lugs look like that contacts the barrel extension nut?

Agreed, it would be good to get pictures on the backside (tail end) of the lugs on that bolt. Maybe some metal smear where it was unlocking as the round went off.

 

[Shrinkmd]

Do you see any damage?

I am very concerned about the possibility. Some of the rounds were overloaded. I’ve successfully made thousands of rounds on my 1050, and stop any time the powder check gives a peep. AA2230 loads to way higher for 556, max load 26.3 with the Hornady 55 gr fmj, and I’m loading 24.5

 

[jmorris]

He thought I annealed a primed case, but I know that didn't happen, my ears would still be ringing,

and the case still wouldn’t be blown out.

 

[243winxb]

Pock marked bolt face not an issue for safety, for me. New owner replaced it. Caused by loose primer pocket or defective primer. Or wrong primer-

 

[Nature Boy]

They look like flame cutting from blown primers. What happens when you overload rounds and start blowing primers.

Or when the brass has enlarged primer pockets due to repeated reloading cycles and the primer no longer creates a tight seal.

 

[Shrinkmd]

Or when the brass has enlarged primer pockets due to repeated reloading cycles and the primer no longer creates a tight seal.

I hadn't noticed before, I will have to keep an eye on that.

 

[BreechFace]

Even an overcharge on a chambered AR15 would have to sheer lugs, blow out the extractor, etc. This was an OOB in a system not designed to be able to happen; so something is amiss.

How many rounds have you put through the rifle as it stands right now?
Did you make any recent changes to the rifle?

Nothing looks out of the ordinary from the pictures of the lugs. But it's always hard to tell from photos.

Be curious to see your measurements and weights on your buffer system, that I posted about in post #65.

 

[Shrinkmd]

Even an overcharge on a chambered AR15 would have to sheer lugs, blow out the extractor, etc. This was an OOB in a system not designed to be able to happen; so something is amiss.

How many rounds have you put through the rifle as it stands right now?
Did you make any recent changes to the rifle?

Nothing looks out of the ordinary from the pictures of the lugs. But it's always hard to tell from photos.

Be curious to see your measurements and weights on your buffer system, that I posted about in post #65.

Maybe about 3k rounds through it, less than 5, I stopped keeping track a while ago. No recent changes, I put the LaRue trigger in more than 2 years ago, and no other changes.

I will take out the buffer and look at the spring later. It is the standard BCM equipment it came with...

In doing all of this, it jogged my memory that my rifle had doubled one other time in the last month or so, but I don't remember seeing the brass case looking deformed. I remember looking at the primer, and it looked similar to this one, with what looked like a light strike on it, I forget if it was bulging out. If I recall, it was also using the Win #41 primers, same batch.

 

[BreechFace]

The doubling your rifle is doing periodically is what I think you should focus your efforts in ascertaining why. That would have the potential as @someguy2800 astutely saw out of your description could cause a scenario of unlocking as the detonation is occurring.

This would be a buffer system problem, although there were initial problems with propellant causing bolt bounce when the M16 was going through it's teething pains if I remember correctly.

Generally, bolt bounce is caused by the carrier moving forward to quickly. More buffer weight will help with this.