This is a textbook example of case failure, caused by a hot load or excessive head space. The key in determining the cause requires analysis of the casing for evidence of a hot load. Usually the case goes slightly molten and will leave deep marks on the rim from the extractor.

Were there several FTF on multiple rounds prior? That is an indication of excessive headspace.

Contrary to what another comment said, headspace IS important with an AR. A barrel and bolt on the opposite sides of tolerance will go out of spec in a hurry. Even the removal and reinstallation of the same barrel can make a rifle out of spec due to changes in angel of the bolt relative to the barrel extention. There is only .0085″ of tolerance allowable in the 5.56mm chamber to be considered safe, so there really isn’t much room for error.

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I’m referring to a low load density and to the powder charge shifting. The way the pressure curve gets erratic. I’m sure you can dig out some data from your testing. In a laboratory where you can measure the pressure, not read primers? I’ve also been reloading and shooting for over 30 years.

Thanks for the insult, always the first thing out of the mouth of you “smarter than the rest of us” people.

This has been talked about for years. I had even talked to Marty Liggins at Accurate Arms Powder about this very subject back in the 90′s. They were aware of it back then. A powder manufacturer. Hmm Nothing mythological about it my friend. But hey, I guess the rest of us are not as smart as you.

I was just throwing in another possibility for the AR failure. As you are aware, ammunition companies are running mad to keep up with demand. Long hours and high speed can lead to issues with ammunition. Any ammunition.

It’s explained here better in this one of many examples:

Hoss Topper hossie@irs.com
wrote:

About low charge detonation in firearms

spread out along the bottom of the case with a lot of airspace left above
it, it can — sometimes — detonate or at least burn MUCH more rapidly.

In fact, it will burn too slowly at the beginning. Later on, the powder
burns very fast.

According to ammunition and powder companies, the mechanism is not yet fully understood.

( Wow even the companies that make the powder and ammunition don’t understand it. Call them Mr. Applegate and clear it all up!)

Meanwhile, reloader’s are advised to use caution.

How come? In think the mechanism isn’t that difficult at all to
understand. If you have a too light load unevenly spread in the case,
the charge will ignite partly, slowly or not at all because of too low
a pressure, too weak a primer or uneven distribution of the flame
across the powder volume. Then the bullet may start decelerating or
even stops in the barrel.

Should that happen, the gases behind the bullet stagnate and the
pressure and temperature rise sharply at the bullet base. As the powder
charge always moves with the bullet, the powder starts burning rapidly
at the bullet base (local pressure peak) and amplifies the pressure
peak. Next, the pressure peak will start traveling towards the bolt in
the barrel and reflects back at the case bottom (bolt face). All the time,
the pressure wave gains more and more energy from the powder spread
along the case and the barrel behind the bullet. When this oscillation
reaches high enough amplitude, it turns into a shockwave, i.e. a
detonation (powder is still present to provide the shock wave with
energy).

[Mr. Applegate says this is a myth]

Such a small charge detonation is always preceded by several pressure
peaks (can be seen in a test barrel with pressure transducers) until
the peaks reach very high pressures and all of the powder still left
burns with a single pass of the wave, which indicates a real
detonation.

fast-burning pistol powders in squib loads for large-cased rifle calibers.

Pistol powders (especially the porous types) ignite easier than coated
rifle powders. In addition, it is best to use as strong a primer as
available for those light loads to ensure proper ignition of the
powder. Also, as pistol powders burn faster, the pressure rises high
enough for stable burning and for keeping the bullet under acceleration
until the powder burns out.

Using pistol or other easily igniting and fast burning propellant is
the correct procedure to avoid a low charge detonation. A filler also
helps, as the powder charge is kept close to the primer and ignites all
at once.

BTW, revolvers are particularly prone to low charge detonation, as the
bullet hits the lands hard and always decelerates strongly.

[I think I described a revolver in my post. Imagine that.]

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http://members.shaw.ca/cronhelm/DevelopSubsonic.html

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OW MY EYES!!!

Oh come on now Jake, that is just so wrong on so many levels. Do you actually believe that? I hope not. If so then it’s painully obvious you know exactly nothing about the internal ballistics of the modern smokeless powder cartridge.

There is no way you can achieve detonation with any currently available commercial smokeless gunpowder in a cartridge as small as the .44 Magnum or the .223 Remington.

The only problem one might encounter as a result of dropping a 9mm powder charge into a .44 magnum case would be the bullet MIGHT lodge itself in the barrel somewhere short of the muzzle.

In my career, I have examined well over a dozen kabooms and in every single instance (where there was not a previous obstruction in the bore), EVERY single one was caused by a LARGER than safe powder charge. As I understand it, so far, no one, anywhere, has ever been able to create a kaboom on purpose in any sort of small arms by simply reducing powder charges.

And there are many who have tried to do so.

Extensively tried to do so.

Myself included.

In 1983, using every then commercially available handgun propellant, I personally supervised testing of reduced powder charges in .380acp, 9mm Luger, Super .38, .38 Special, .357 Magnum, .41 Magnum, .44 Special, .44 Magnum, .45acp and .45 Colt. And I do mean EVERY commercial powder, as well as several non-cannister powders, suitable for handgun use available in the US, from the fastest burning to the slowest burning in EACH of the aforementioned calibers. Using several bullet weights in each caliber to boot. Now lemme tell you, that was a heck of a lot of testing.
For each caliber/powder/bullet combination, I lowered the charge weight until the bullets would no longer reliably clear the barrel.
Wanna guess what happened?
Not a damned thing.
There were absolutely NO indications of any sort of excess pressures.
Without fail, as charge weights were lowered, velocities simply became slower. Until they became too slow to overcome the friction of the bore.

Even with the dreaded H-110/WW296 (and 297) there were no kabooms. Not even close. All of the reloading manuals caution against using reduced charges of 110/296 (I use the term 110/296 because the two powders are exactly the same-from the from same formula on the machinery), beacuse there IS a problem with using reduced loadings of 110/296. That problem however is NOT detonation. What happens is that one shot will go off just fine while the very next just might leave you with a bullet stuck halfway down the bore. Now if one were so careless as to execute a double tap with one of those non-recommended reduced loadings one could very well have a problem. But then no one in their right mind chooses 110/296 for reduced loadings anyway right? After all it was designed for full-tilt-boogie power levels.

So here we are, nearly a full decade into the 21st century and yet there are still untold numbers of Chairborne Rangers out there that insist on perpetuating these urbane myths about mystical exploding powder charges. And it’s all just so much yellow precipitation down our necks to shift focus away from careless handloading.

Face the facts Jake, your IPSC buddy simply double charged a case and didn’t want to man up to the truth that it was 100% his fault.

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You should have just bought yourself a new upper.

Colt is never going to admit to wrong doing, Colt will come to the conclusion it was the ammo or etc, etc, etc.

If their at fault you get a free upper and they will push the blame as above, so either way you’ll get a new gun.

Just my 2 cents worth!

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Shooter lying on his right side.

Ejection port facing toward ground.

Ejection port very close to ground.

Ground has small bits of rock.

First shot causes bit of rock to enter ejection port.

Bit of rock lodges between boltface and primer of second round.

Bolt comes into almost-battery with rock still lodged against primer.

Trigger pulled.

Hammer strikes rear of firing pin.

Firing pin cannot reach primer because of out-of-battery status of bolt.

Force of hammer striking firing pin drives bolt/carrier assembly against rock.

Rock is in contact with primer.

Bolt is out of battery.

Primer fires.

Round discharges out of battery.

Possible?

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Someone was arguing that the AR was able to fire out of battery. Nate cut his bolt out and took pictures to show it can’t. The bolt must be fully close in order for the firepin to reach the primer.

http://www.m4carbine.net/showthread.php?t=30053&page=5

As with the other two cases the AR was feeding XM193.

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There are no cast upper or lower in the market today IIRC. There were before 2000 but not anymore. All upper and lower are either made with Billet (6065-T6) or Forge (7075-T6). Of the two the forge is stronger and it is what the vast majority of AR15 receivers are made off.

Keith Applegate,

Yes that is what he meant. There have been reports of problems with the XM193 for sometime.

http://www.thegunzone.com/stag15_lc.html

Here is another AR15 that got blown up by XM193 ammo.

http://www.ar15.com/forums/topic.html?b=3&f=16&t=440162

Always wear safety glass when shooting.

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I wonder if he means, by any chance, Federal Black Box/Tan Box XM193?

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From the looks of it, I would say that that failure mode would be safer for a left hander!

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Take your BCG out.
Compress the bolt (locked position, in battery).
Depress the firepin from the back.
You will notice that fire pin is sticking out of the fire pin hole.

Extend the bolt (forward, this is the unlock position, out of battery).
Depress the firepin from the back.
You will notice the fire pin isn’t sticking out of the fire pin hole. If the fire pin can’t strike the primer that means the round won’t go off.

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AR15 bad bolt are always associated with the locking lugs or cam pin breaking in few hundred rounds.

Out of spec chamber is associated with failure to feed.

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