Most likely scenario is he mixed in a .300 Blackout round with his .223/5.56 ammo and...kaboom. Happens far too often. Then it's always "no way did I do that". But...let's see the empty case. Well looky there....300 BO.
It doesn't help that people cut down .223 cases to make .300 Blackout cases so the headstamp even says .223 making it just that much easier to make a mistake of putting a .300 AAC BO round into a .223 magazine without noticing. And when they get the bolt open after the destruction, the case may even say .223 on the headstamp (if it is still readable) but it was a .300 BO round in a .223 case. It makes it so easy for people to blame the gun or "factory" ammo when it was "user error". We're all human (even weirdos that dress up and play commando on Youtube.) Mistakes happen. Gotta be REAL CAREFUL if you own both a .223/5.56 gun and a .300 AAC Blackout gun.
LFD Research simulated a catastrophic failure by chambering a .300 Blackout round into 5.56 AR-15 and filming it in slow motion.
www.thefirearmblog.com