Ponder this.

Go back and look at the picture.

Look at the carbon on the head of the case by the rupture. It is half moon shaped

All the splits are linear. All seem to originate just ahead if the extractor groove in the same point on the half moon shaped carbon stain. the cracks them selves go three different ways.

How does carbon stain the outside of brass? By flowing between case and chamber wall. How can leaking gas create such a shaped stain.

What kind of gun was this ammo fired in anyway?

I think we all are assuming AR here.

Read this.

" As you might imagine, quite a bit of safety tolerance or “fudge factor” is built into the systems, everything from extra strong steel and gas blocks in the bolt raceways to gas shrouds on the ends of the bolt body and escape vents (those holes in the front receiver rings.) "

And,

"This was a well built, highly respected factory rifle and factory ammunition."

Sounds more like a bolt gun, to me.

Wonder if some one cut a feed ramp on the barrel. That's the rookie stuff we did at school when we didn't know how to adjust feed rails and magazine followers. This was usually followed by Instruction from the "Colonel" as we knew the professor to set the barrel back and do it right.

Just pondering and speculating like the rest of you.