300 Blackout in a .223 Wylde AR-15 Kaboom

One of the huge draws of the 300 AAC Blackout (AKA 300 BLK) is that it can use the same bolt and magazines as a .223/5.56 AR-15. The only downside? It can use the same bolt and magazines as the .223/5.56 AR-15.

A friend and reader presented me with a box of AR-15 upper receiver parts this past weekend that vividly illustrates what happens when a distracted or negligent person loads magazines.

A single 300 BLK cartridge somehow found it’s way into a box of loose .223 cartridges, and was subsequently loaded into a 30-round PMAG, which was then handed to an AR-15 shooter shooting a rifle chambered in .223 Wylde (a chambering that is supposed to shoot both .223 Remington and 5.56 NATO equally well).

The .223 cartridges in the magazine went “bang-bang-bang.” The 300 Blackout round apparently fed into the chamber just fine, the shooter pulled the trigger, and what you’ll see on the following pages were the result of the ensuing kaboom.

Luckily, the shooter was shaken, but uninjured.


photo 1

This is the remains of the upper receiver, as I’ve been able to partially reassemble it. You’ll note that the part below the ejection port has been destroyed. The front matter (barrel, gas tube, handguard, etc) with the cartridge and bullet lodged inside, remain with the owner. He is going to attempt to salvage some of the parts, along with the lower, which was also salvageable. The upper receiver, charging handle, and bolt carrier group are a total loss.

You’ll note that I was able to get the bolt back into the upper receiver for the photo, but that there are parts missing. Some were destroyed during the kaboom, and some during the process of prying things apart.


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When they were able to pry the bolt carrier group out of the upper and remove the bolt, the cartridge case head came with it, while the rest of the cartridge case and the bullet remained lodged in the barrel.

photo 2


photo 1

This split/fractured bolt carrier seems to have absorbed the bulk of the internal damage, and both it and the bolt are now scrap.

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The rear of the upper was also split, but I don’t know if this was from the kaboom or the salvage process.

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As a matter of simply physics, explosions follow the path of least resistance, which in the AR-15, is typically down the magwell. This is reflected in the condition of the 30-round PMAG that was blown out of the rifle.

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Here’s my big question: is it normal for a 300 BLK cartridge to chamber well enough for the bolt to close under normal conditions in a 5.56 NATO or .223 Remington chambered AR-15 rifle, or is perhaps possible that the rifle fired without the bolt fully closing? I’m sure there are readers out there far better equipped to answer this question than I am.