Several thoughts.
Case head failure.
Bolt carrier failure due to improper Metallurgy.
Early discharge resulting in unsupported case head.
One other thought.
God and his Angels Hover over our Grunts.
I'm Glad this Doggie got a sore nose, and nothing more.
The bolt is locked into the Barrel extension, either partially or completely.
Pressure was dumped directly into the upper.
Gas tube and handguards are intact.
(Failure occured before gas tap was under pressure)
Bolt carrier is obviously TOAST, and possibly indicates a pressure path back through the primer when the split upper is considered.
Which of course leads one to think about an obstructed bore.
Whatever.
The rifle failed, and did so in a manner that kept the Kid alive and just a little rattled.
Bad shit happens.
I'm glad the Kid didn't opt for a lesser rifle.
S-28