Kaboom!!! 300 BLK in 5.56/223 AR-15!

Some unfortunate shooters have learned the hard way that is is possible to fire a 300BLK in a rifle chambered for 5.56mm or 223 Remington. The results of such a dangerous event are spectacular. In this post, we’ll take a look at a rifle that detonated as a result of this.

My friend was a first hand witness of the incident. The shooter had a mix of 5.56mm and 300 BLK ammo present. He unintentionally loaded a 300 BLK into the 5.56 magazine. When he was firing the rifle, the 300 BLK round chambered, fired and the gun detonated. The shooter did not have to hit the forward assist or force anything into place, the 300 BLK round simply cycled into the chamber.

If you search around the Internet, you’ll find some drawings that show how this can occur; basically, the bullet in the 300 BLK cartridge is forced back a bit during the chambering process and headspaces off the 5.56/223 chamber allowing the firing pin to set it off.

This event seemed to be more severe than the critical failure of a Rock River Arms rifle I personally witnessed in 2012 (click here for that post).

In this post, we’ll take a look at the damage caused by an event like this. Fortunately, no one was injured and when you look at the pictures below, you can tell that the shooter was extremely lucky.

We’ll start with the image of the 300 BLK slug that was fire formed in the bore. I used my mill to cut down the barrel. On the bottom right of the barrel, you’ll notice some rough spots that were caused when I abandoned the first set up to cut into the barrel and ended up switching how I held it. It wasn’t caused during the event. You’ll notice the round made it quite a distance, roughly midway between the base block and barrel extension on this carbine-length gas system upper.

I’m still trying to confirm the round that was fired; however, after it was fire formed in the bore, it is certainly longer. I measured it at 1.648″.

The outside diameter of the barrel did not deform or expand. It was concentric within .001″ throughout the area under the hand guard.

The fired 300 BLK case remained in the chamber when the case head blew off. The barrel extension and barrel nut both spun around the barrel freely.

The front part of the upper receiver, barrel nut and barrel extension were removed. The barrel extension was clearly damaged. The pin that indexes the barrel extension sheered off inside the extension.

Further inspection of the barrel nut show a crack down its length aligned with the center of the left feed ramp cut.

I sectioned the chamber end on the milling machine. When I cut it open, the end mill pulled out the fired 300 BLK case, however you can see where it was when it was fired. The bullet was seated against the tapered part of the chamber just forward of where the case was.

What remains of the separated case head.

The upper receiver split into four pieces, the one piece stayed attached to the barrel and was removed in the images above. Each side blew out and the rail separated cleanly from the left side of the receiver. Note the bend in the ejection port cover.

The bolt carrier split lengthwise and the bottom peeled off. The extractor bent and separated from the bolt. Note the deformation in the last gas ring.

The bolt carrier with the bolt removed. Somewhat surprisingly, the gas key remained intact.

Note the hole in the extractor was compromised and the claw of the extractor bent backwards.

A side view of the bolt and bolt carrier group. Notice the piece of shrapnel that peeled off the bottom.

The rear lug of the upper receiver sheered off.

Most of the lower, besides the magazine and magazine area survived the event. In the front of the lower, the side of the magazine well flared out.

The hook on the charging handle latch and the extended handle both sheered off.

The bottom mounting area of the MAGPUL BUIS broke off.

The shooter and those accompanying him at the range were lucky to have survived this event without injury. I share these pictures as a cautionary tale about the importance of paying attention to what you are doing. In the shooting sports, a few moments of inattention can have lasting effects.

If you like looking at damaged rifles, click here.