Bad brass
That break looks the same as being cut with a fine tooth saw or file -no case separation caused by excessive headspace, which would be indicated by separation in front of the internal cartridge web, a strong work hardened portion. A real clean even separation in front of the extractor groove, the strongest portion of the case. Not a case separation caused by excessive head space.
My thinking is:
The smooth & precise separation was the result of a defect. Various materials, in the plastic state, show signs of failure or separation when subjected to force directed normal to the failed area, called a "stress fracture". My guess is that this failure was initiated when the case was made. The head area is work hardened and when the case was made by forming plastic state brass, a stress fracture was initiated, and the case failed later when more normally directed forces were applied, like in chambering a crush fit round to fire form the Ackley Improved brass case - a .004 or more crush fit.
Location of the brass in chamber has no effect on the subject separation as shown by imminent failure on non-separated areas, like shown in internal photos. Any exposed brass would be located away from internal high pressures near the bolt face with extractor groove exposed. The failure occured in front of the extractor groove surrounded by chamber walls.
I would guess that parts used for critical stuff, like life & limb, would be subjected to various tests like X-ray to look for any defects.
