Well I've seen very similar results happen from factory ammo.
My theory (without seeing the gun in my hands, and I'd love some ammo to run thru a pressure barrel with a strain gauge), is that you have ringed your chamber near the bolt face end of it. Do you have any of your previously shot brass?
Primarily I've seen this on old SR-25's shot with M993 AP ammo (which is way above proof pressure when loaded by NAMMO).
I recently saw a SR-25 have the same issue with some factory SSA 175gr. The rounds where all over pressure (which in your case may have been caused by bullet setback) this is turn affected the way the bullet was sealed (or rather not sealed0 in the chamber. The gap in the ringed chamber allows the high pressure gasses to blow out the side of the casing into the gap.
Anyway just my 0.02 from sitting back and looking at the pictures.
As Suputin and several others have said this was NOT a round fired from an unlocked gun (it would be very visible in that kaboom).
The casing just just not fully supported by the chamber wall in some area.
Even when casings fail (I've got a slew of 60gr TAP that hornady screwed up the brass on - I think some Calgary folks saw the the results on the brass and my Colt LE6921 chamber of that. In that way the casings where cracking and the chamber got flame cut in certain areas - I noticed it early on, and #####ed and moaned to Hornady. But for damage to happen to the casing that bad, there had to be a area of the casing that allowed for room for the casing to blow out like that.
Generally I only use a powder that even with a compressed load (from something like bullet setback) will not cause drastic overpressures (I like Varget for 5.56 and 7.62mm guns).
However I will say its not an XCR issue -- but I would get a chamber cast made of the barrel - to ensure that it has not been ringed (which I think it has), for it it has, your going to end up with a very similiar issue in the future.
