I'll just bet it was a round fired over a squib. My first AR's upper got poofed out when my buddies GF was shooting it. I was well back from the line loading another mag with my back turned. When I heard a very slight 'pop', and didn't get spun around before she'd jacked another on top and ,,, BOOM! The 'pop' didn't register in my head quick enough what it was in time to do anything.
No personal injury, Thank GOD.
If my buddy had been the shooter, the palm of his hand would have been lunch meat. He always had a habit of holding the bottom of the mag instead of the fore end.

The bolt shattered, the remaining cartridges in the mag were FLATTENED, the ammo, follower and floor plate spewed out the bottom of the (now) puffed out mag walls.
The entire mating surface of the upper was then an easy 1/8" wider than the lower at the edge almost all the way around.
The only salvageable parts were little things like the sights, etc.
Not a good day.:jedicrying: