The parent case of the 300BLK is 5.56. While you would think that a BLK cartridge couldn't chamber in a 5.56 rifle it can- at least enough to KB an upper receiver as pictured above at a range in Georgia. Fortunately no one was hurt- but this is important to understand and prevent because the next person may not be so lucky.
While the round probably didn't chamber, it got far enough that the bolt started to close and pushed the projectile backwards into the cartridge- basically smashing the charge and creating enough friction to set it off.
Scary? Yes but this is also easily avoidable:
- Use separate, marked, dedicated mags for both BLK and 5.56
- Some of the best ideas I've heard include:
- use a different color mag for each caliber – like forest green for 5.56 and flat dark earth for BLK.
- use 20rd mags for BLK and 30rd mags for 5.56
- use mag sleeves for one caliber
Find a way to clearly mark your mags or make them so different that you can't screw it up- no one wants to be picking tiny slivers of aluminum out of their face.