During late may I was shooting my Olyarms 7" AR, from the hip (I never do that, but I was a little bored..) when I heard a LOUD bang and the magazine was sent forcefully flying downward. Luckily it was the last round left in the mags. I putted the smoking AR on a table and start examining it: the bolt was stuck in the closed position, the magazine was dinged slightly at the front lip, but was mostly ok.

At home, with the aid of a brass dowel I pushed the bolt until Unlocked the action, and find the following damage:

-BENT EXTRACTOR CLAW:
http://www.imagestation.com/picture/sraid73/p38d6fcdafc880271681bcf9c83c648a9/fb693159.jpg

-BROKEN BOLT CARRIER
http://www.imagestation.com/picture/sraid73/pbb30f93e2d45ade3f053269549b4e315/fb69317c.jpg

http://www.imagestation.com/picture/sraid73/p52cc4b4b7b28ced4477b220d4447c526/fb6931be.jpg

http://www.imagestation.com/picture/sraid73/p852888ea0cf6e4a24218a22450ef8808/fb6931e6.jpg

The last image was the recovered brass, it was blown off at the base.

Upon close inspection, the bolt was perfectly intact, the barrel was not bulged of chipped, all the upper was perfectly ok, the lower also was detail stripped and found in perfect order.

The only minor damage was in the barrel extension lug corrispondent to the extractor claw, it was slightly dinged (say 0,5mm).

During shooting I was firing rapid-fire, and it was roughly the 120th round without pausing, the barrel was very hot (I was holding the gun by the front of magazine).

Upon further investigation I found the the primer on the brass exploded was  SMALL PISTOL one, I don't know how it was possible, but somehow it slipped in my press....

The brass was a military surplus one, it was roughly the 4th reload with 22gr of N120 and a lapua 52gr ball.

Right now I have a new extractor and bolt carrier, and I'm in the process of CAUTIOUSLY test firing the thing,

Anyone has any info on why my ar kaboomed?

Thanks.

I'm have no experience with reloading but I just had a guy come in the with the same kB. Using his reloads, he figures they were on the 3 or 4 loading, with military brass. His brass showed no sign of high pressure but it looked like the brass was brittle. Other cases showed cracks in the same area and around the neck. There was no bulge in the brass that would point to an unsupported case head and he fired 500 rounds of SA ball the next day in class. HS was fine before and after the kB. His rifle did it TWICE on the line during class, blew two mags up.

Could you post a clearer pic of the base of the case plz?

[edited to add request]

I don't know if it was the cause, but there is a good possibility that the primer contributed to the problem.

Pistol primers are typically softer and easier to detonate than rifle primers, and it is possible you had a slam fire. The firing pin on the AR is not restrained by a spring, so it moves and will slightly dimple a rifle primer when the bolt travels forward and locks up. This movement of the firing pin has been known to be sufficient to detonate pistol primers. If this was the case, it may have detonated before the round was completely chambered and before the bolt had fully engaged the locking lugs.

If you were firing slowly, you would have noticed because it would have fired without pulling the trigger, but in rapid fire, it could have happened without you noticing.

Thanks for the answers, guys, here is the recovered brass, side view:

[img]http://www.imagestation.com/picture/sraid73/p287ec1f20ecc2cdf49a1f72486f6d621/fb683e99.jpg[/img]

another view:
http://www.imagestation.com/picture/sraid73/p373cf567f71b8ad5781128e7ff2389dd/fb683f24.jpg[/img]

and another:

[img]http://www.imagestation.com/picture/sraid73/pccb8765cf616feb36ba025a88aadd131/fb683dd4.jpg[/img]

here are some recovered brass, primers are somewhat flattened:

[img]http://www.imagestation.com/picture/sraid73/pdf85a5eb66a0cc6260c3ebc8848c9515/fb683cf4.jpg[/img]

and here is my short barrelled AR:
http://www.imagestation.com/picture/sraid73/pda1cfb4e87c3e017d1a22a792fd240a3/fb683c49.jpg[/img]

Is there someone that can tell me if I've had a blown case or an out-of-battery KB? or maybe a slam-fire.

thanks for info.
[img]
[img]

damn! I screwed posting images...

ok, here they are:

[img]http://www.imagestation.com/picture/sraid73/pda1cfb4e87c3e017d1a22a792fd240a3/fb683c49.jpg[/img]

[img]http://www.imagestation.com/picture/sraid73/p373cf567f71b8ad5781128e7ff2389dd/fb683f24.jpg[/img]

and those:

[img]http://www.imagestation.com/picture/sraid73/pccb8765cf616feb36ba025a88aadd131/fb683dd4.jpg[/img]


[img]http://www.imagestation.com/picture/sraid73/pdf85a5eb66a0cc6260c3ebc8848c9515/fb683cf4.jpg[img]

argh: damned pictures...

[img]http://www.imagestation.com/picture/sraid73/pdf85a5eb66a0cc6260c3ebc8848c9515/fb683cf4.jpg[/img]

Ok, I'm seeing what looks like a definite "belt" forward of the extractor groove on that case. I still can't see the case head clearly to look for pressure signs.

The belt shows that you either had a high pressure load or at that the case was unsupported for some reason.

Is that an OAI barrel? They have problems with their chambers and CAN have too great a distance between the inside of the extension and the rear face of the barrel. Sometimes if the radius leading into the chamber is overdone the same symptom will show up.

Given that this round was the only one to kB and the round is out of spec (primer) I don't really know what to tell you, it could be the ammo or the gun. Has the HS been checked on the barrel?

Slam fires are an option but slam fires don't cause kBs, they're just an unpleasant surprise for the operator. The M16's firing pin cannot reach the primer until the bolt is closed and locked.

Blown bolt carriers generally mean high pressure loads, I don't know if a pistol primer would drive the chamber pressure up tho.

Did the brass extrude into the ejector hole? you'll see this as shiny, circular, mark on the base of the case.

Have you pulled rounds from that same lot and checked the powder and charge?

[ed for speeling]

I agree with Neilfj. It appears to have been a slamfire. It has the unsupported base that wasn't all the way into the chamber and less damage than a high pressure failure (although still spectacular). The M-16, M-14, and M-1 Rifles all have a floating firing pin. Although all three rifles shouldn't allow the hammer to drive the firing pin until the bolt is locked, all will have an indent from the firing pin when the bolt slams home. With the muzzle pointed in a safe direction, hit the bolt release and chamber a round and then extract it and examine the primer. It will have a firing pin indent. Pistol primers only have to deal with 10 to 15 thousand psi unlike 50,000 psi for a rifle. Therefore, pistol primers are made much softer and should never be used in rifle loadings. It would be good to know if the round chambered and you pulled the trigger or if it went off upon chambering. Probably the latter. From what you say and show, I would think that it was a slamfire due to the soft pistol primer.

Thanks for info, guys,

As I've said, the lug on the barrel extension is chipped by a very small amount, would it be dangerous to test fire the rifle? the chipped lug is the one at 2'o clock, th one that lock the extractor.

You may want to have it MagnaFluxed (Magnetic Particle) inspected. Hey, you already did the proof load test, you might as well go whole hog. [;D]

I've had my ar checked by a gunsmith and he said that the chipping on the extractor lug was really minor and everything else seemed ok, no signs of cracks or metal stress anywhere so I bite the bullet and test fired it...damn, I was wearing double shooting glasses, a scarf and gloves...My buddies were cracking in half from laughs..however, I shot 100 rds of winchester factory ammo without blowing up myself so I consider it a good sign...:)