Had a very soft case head been the cause, would not early pressure signs from a lower charge weight been apparent? Certainly if the head and web of the case were soft, a sticky bolt lift would have signaled a problem.
Without being able to subject that lot of brass to a Brinell test or similar, I can only go from the photos. The 4th head has a dramatically-enlarged vent and primer pocket, which suggests that the metal has flowed under pressure (duh). To me, all other things being equal, that suggests a softer metal rather than a harder. In any case, assuming you have used that particular casing before, the problem is clearly one of a huge spike in pressure, not the brass.
WRT the suggestion of pressure detonation, there is of course a difference between that and simple overpressure caused by too high a loading or by an obstructed barrel (I assume you're sure that the barrel was clear for that shot). The damage I've seen from detonations (on a pair of SA80s, as it happens) is considerably more, shall we say, 'impressive', so without discounting it entirely, I'd be less likely to attribute this to detonation.
One thought - a number of reloading sources note that different bullets of the same weight can result in differing pressures. The jackets of two brands of bullets or even two different designs of the same brand may be of a different hardness; the harder one would of course raise the pressure to some degree. Moreover, due to the shape of the bullet, one bullet may have more jacket in contact with the barrel, raising pressure.
Another - Julian Hatcher noted that the Yank national rifle matches had a string of breech explosions which he traced to the then-common habit of oiling the ammo. Is it possible that that round was still lubed from reloading?
Due to the chamber dimensions and the long barrel of my rifle, .308 book data, as it relates to maximum loads and maximum velocity, isn't relevant. For example, the Sierra manual states 2450 fps is the maximum velocity for a .308 with a 200 gr bullet backed by 45.5 grs of W-760, yet my rifle can drive a 200 gr MK to 2660 with 50 grs of W 760. My proven loads have much more in common with a .308 AI than with a SAMI .308. When I looked at the Lapua data for the 190 with N550, I figured that 47 grs should be close to maximum in my rifle with the 210, which turned out to be accurate. Yet I do not hot rod my loads, when I have pressure signs on my brass, I back off a full grain and consider that to be my maximum working load, then work down from that point to find the best accuracy.
You seem to have approached your loading workups with a scientific approach and clearly have much experience. Yet - and no disrespect intended - I would not be comfortable trying to stretch the envelope without a proper (and yes, expensive) pressure rig. I note that you have exceeded the Sierra recommended max load for that 200gr/W760 load by close to 10%, yet don't consider that 'hot-rodding' because you have not been able to discern pressure signs on the brass. Those classic warning signs are just that; they're not scientific pressure evaluation methods.
I guess what I am trying to say is that the reason the old X-series test pilots had short lifespans was because they were constantly trying to push the envelope. The closer to the edge you get, the more likely you are to fall off. Again, no finger-pointing, but it would seem that you have not only been living on the pressure edge, but spent some time leaning over it.
Glad you're OK.
