Im really proud the shooter took the time to close the scope cap
Ive heard it was from a bad batch of stainless steel, with a high sulfer content. I think Tikka? had a similair problem with a barrel or two on their rifles banana peeling .
Ive chatted with a guy who used to work for Barrett a few times, and he has experienced several failures with .50BMGs. He said it felt like a grenade going off, and made him sick from the pressure wave.
Ill bet that left the shooter flinching for a little bit.