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Impressive AR Over-Pressure Event

Saying, ‘over-pressure event’ is a polite way of saying, “KaBoom” (or ‘kB’ for short). These can happen with any weapon if improper ammunition is used or a bore is obstructed. The AR kB’s tend to make the rounds a little more both because of the higher number in circulation and how rabidly the anti-AR crowd proliferates them. In this case we’re doing it to educate, and by God were the pictures taken after well done.

The back story of this particular kB comes from Justin Colter and was originally posted on the M4Carbine.net forums. An Army veteran with more than a decade of experience, Justin is a co-owner and instructor for Arma Dynamics out of Indiana. He also takes some fine photographs, more of which can be glimpsed on his Flikr page. Let’s look, read up, and consider possible causes.
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A friend of mine experienced an AR over-pressure ‘kaboom’ with his 3 Gun rifle yesterday. We’re thinking it was an over-loaded reload that led to an obvious over pressure.

He is fine and only had his face blackened by the gas, no injuries.

The upper is toast, but surprising the lower seems to be perfectly okay. No visible cracks or any other signs of deformation or damage. We installed a different upper to check fit and everything went together fine with no binding and everything functioned. We did not fire it as a more thorough inspection is probably warranted.

The rifle was built on a Seekins billet receiver set with a JP low-mass BCG. The barrel was a Nordic 18′ SS .223 Wylde chamber.

Loads were 55gr Hornady FMJs over 26gr of Ramshot TAC. No possibility of pistol powder being a factor. The only other possibility is that a range pickup from a match may have made it’s way into his ammo. I’ve run that exact load for thousands of rounds through my 5.56 guns with no issue whatsoever.

  • Bolt sheared in two at the cam pin hole, and split the tail in two.
  • Bottom of the carrier split and peeled away
  • Upper receiver cracked and split in multiple places
  • Rear upper receiver to lower receiver lug sheared off
  • Magazine blew apart
  • Bolt face was stuck in the barrel along with the chambered brass
  • It looks like the bolt just started to slightly rotate out of the barrel extension before everything went

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[Click on any photo to start gallery view]
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There are a few things to keep in mind: Regardless of the cause of a kB, the AR is specifically designed in such a way to keep pressure away from the shooter. That isn’t to say that injuries (or worse) cannot or do not occur when an AR goes boom–it’s just much less likely than with weapons of similar vintage. Parts and pieces can become secondary projectiles and a breeched barrel can do a number on your support hand.

Take note of the bulged ejection port and the magazine. These are common places for the pressure the rifle is subject to in an over-pressure event to express. Had it not been a billet upper, it is quite possible that the left side of the upper receiver would be cracked or swelled (in extreme cases it banana peels from the front). Since the barrel appears to be intact, Justin’s initial assessment that the reloaded ammunition used was to blame is probably correct. The shell casing is also exhibiting classic signs of over-pressure. Though we can see a broken bolt (sheared at the cam pin hole), we’d like to see if the bolt face itself was cracked too.

Anyway, what say you? Let us know in the comments.

For some previous discussion and photos of kB, check this out.

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