Well, Slamfire, I guess I have to bow to expertise as you have had and seen more slamfires than anyone else I have ever known.* But in my much more limited experience, a slamfire with no backup to the firing pin (that is nothing but firing pin inertia) will let the primer internal pressure push the primer metal back out, even into the firing pin hole if the design allows it. It takes a goodly amount of firing pin inertia and case support to get that kind of primer appearance.
The high primer type slamfire is the result of the bolt face hitting the primer; there usually is no firing pin mark at all since the firing pin didn't set off the primer, the bolt face did.
The FN-49 and FAL both have very strong firing pin springs just to prevent firing pin creep and possible slam fires, since the bolt is moving straight forward and no inertia is taken up by the bolt turning.
I still think there is something about that KB we don't know.
*Nine times out of ten when someone says "slamfire" they mean the bolt was locked but the sear jarred off or a light trigger was pulled accidentally, but that certainly is not the situation when a receiver blows apart.