BCC

My best guess is still a blown primer, although the fluted FP should have prevented the damage. Often the force is channeled back thru the FP hole and drive a non-fluted FP like a piston into the rear of the FP retainer and break the rear of the breach block. In this case and others I've seen, it appears that the force traveled upward. Maybe someone here can offer a better explaination but I don't see any way that the extractor would have (in normal operation) had enough force to do this, esp. when you consider it is moving up againist a lot of spring tension.