Thanks for all the ideas and suggestions so far.
Here is some more info:
The rifle has a carbine length gas tube.
The bolt was closed (lugs locked behind extension) after the event.
The BC was pushed slighly backwards but not enough to unlock the bolt.
There is actually enough space available between barrel and bolt for the "homemade Weatherby belt" due to the generous radius on the barrel end (see picture).
Blown primer, belt on case head, and extrusion of case into ejector hole are clear signs of EXCESSIVE pressure (much more than "blue pill" proof loads).
Where did this pressure come from?
Threaded Mode