The .300 Blackout and the .223 Rem. can be a dangerous mismatch
The .300 Blackout is a perfect fit in the AR-15, using the same bolt and magazine as the more common .223/5.56 models. With a lack of caution on the shooter’s part, those attributes can prove a dangerous combination.
Last year, a friend of mine called and told me his company had blown up one of their high-end AR-15 rifles during testing. On the range, they had two rifles, one an AR15 with a .223 Wylde chamber (a hybrid .223/5.56 chamber) and another in .300 Blackout. During shooting, one of the magazines loaded with .300 Blackouts was inadvertently loaded into the .223 Wilde, the action closed from an open bolt and the forward assist used. The result was a destroyed rifle and no one hurt. The .300 Blackout can headspace in a .223/5.56 chamber and the results are disastrous.
Headspace is a simple concept that can seem complex at first glance. The textbook answer is that headspace is the measure from the breech face to the portion of the cartridge that stops its forward movement into the chamber. Examples make it easier to understand. On a rimmed case, like the .22 Long Rifle, it is the front face of the rim that sets headspace as it is trapped between the barrel and the breech face. Headspace on a .45 ACP is the distance between its case mouth and the breech face. Belted magnum headspace is set between the belt and breech face, while bottleneck cartridges headspace on the case shoulder.
The .223/5.56 cartridge is the parent case of the .300 Blackout but they do not share the same headspace dimensions because of differing case lengths. The .223/5.56 headspaces further forward than the Blackout, but they do share similar overall cartridge lengths because of the Blackout’s much longer .30 caliber bullet. An empty Blackout case will not headspace in a .223/5.56 chamber, but when loaded with bullet, especially a soft lead bullet, all bets are off.
When my friend’s rifle went into battery with the Blackout round, a couple of unusual events were forced into motion. Thrust forcefully into the chamber by the buffer spring, the bullet was rammed not into true headspace, but into the neck portion of the .223/5.56 chamber. Use of the forward assist completed the task, either deforming the bullet or pushing it back into its case. Whichever occurred, the locking lugs engaged while artificially headspaced on the bullet. Once fired, many things happened nearly instantaneously.
With the bullet plugging the chamber, pressure spiked dramatically causing the case to fail. High pressure gas flowed through the paths of least resistance, the extractor and firing pin raceway, and finally into the AR15’s aluminum receiver. Both sides of the receiver blew out and the mag well flared, blowing out the magazine. The lugs did not unlock and did not shear, preventing serious injury. The rifle was a complete loss.
This isn’t a story about an inherent failing with either the ammunition or the AR15 rifle. If anything, the rifle performed extraordinarily well handing this high pressure event. This is a story about carelessness and a call for caution regarding the .223/5.56 chamber and the .300 Blackout cartridge. They fit in the same magazines and use the same bolt face. It is up to you guarantee the right cartridge goes into the chamber.