This shooter was lucky. If that hadn’t been a bull barrel that thing would have split apart like a banana. Apparently the mag blew out and the firearm didn’t go unpunished.
I have to assume the cartridge chambered by driving the bullet deep into the case. That would certainly be possible with sub-sonic loads, which leave a lot of case space unused. That would also help prevent an even more catastrophic failure.
Still, pretty cool to see the bullet all stretched out like that.