There were more questions raised than answered in my post yesterday on the 300 AAC Blackout kaboom in a 223 Wylde-chambered AR-15, so I thought I’d do some more research.

I started  last night by grabbing a 223 Wylde-chambered AR-15, which was easy for me since that is the chambering of my personal AR-15. For those of you not familiar with the Wylde, it is a modified chamber designed with the intent of handling .223 Remington and 5.56 NATO cartridges with equal success. Like the Noveske Match Mod 0 chamber, it is designed to work with longer and heavier bullets .223/5.56 bullets in the 77-80-grain range favored by longer distance shooters.

My second task was locating Remington-branded factory 300 AAC Blackout ammunition. I was able to find a box each of both 115-grain closed-tip flat base (CTFB) and 125-grain Match King Flat Base (MKFB) cartridges.

I loaded one of the 115-grain Remington CTFB 300 AAC Blackout cartridges into a standard-capacity 30-round GI aluminum magazine, inserted the magazine into the mag well, and allowed the bolt to run forward. The bolt did not close, and while I didn’t have a caliper handy to measure the exact distance, it seems to have stopped short about 1/4″ from chambering.  I ejected the round, and noticed the feed ramp had scraped the jacket of the 30-caliber bullet. I then replicated the same process with the 125-grain MKFB cartridge. It also failed to fully chamber, and left a scrape on the .30-caliber bullet from the .223 feed ramp.

At no point did I attempt to fire either cartridge, because, “duh.”

This morning I called Advanced Armament Corporation, creators of the 300 AAC Blackout cartridge, and spoke to Brent. While he didn’t have the photos of “kaboomed” upper in front of him, he suspects that the rifle might have fired out of battery, and/or that the bullet most likely was forced back into the cartridge case for the rifle to fire, and/or that the bolt itself was perhaps out of spec.

The “lessons learned” should be obvious, but are worth re-iterating anyway.

• Always make sure that you use the correct ammunition in your firearms.
• Always load your own magazines, as the person most directly affected by out-of-spec or wrong ammunition will be you.
• If a cartridge does not chamber, it is probably better to eject that round than use the forward assist (It didn’t happen here, but better safe than sorry as a general rule).

Brent of Advanced Armament was unaware of this sort of kaboom happening before involving a 300 BLK in a 223 Wylde.

It should not have happened with an in-spec bolt and properly crimped ammunition, even if the wrong ammunition is loaded in the magazine. Unfortunately, the damage to firearm and cartridge during this incident and subsequent salvage attempt means that we’ll never know the exact cause.

Be safe out there, folks, and pay attention.