Is there any damage to the right rear lug on the bolt? From the damage to the receiver's right lug recess it looks to me like the round went off with the bolt engaged only about half way.
My experience with case failure is that the round will fire just fine, cycle but only eject the rear portion of the case leaving the rest of it stuck in the chamber. Most of the case-head separations I've seen happened where the case wall meets the head of the case. As such when it happens there is little chance of a second round being chambered to the point where the hammer could ever engage the firing pin let alone to the degree the bolt would close to the extent it looks like this one did.
My vote goes to out-of-battery detonation, caused by either a high primer, seized firing pin, or hammer follow.
Would you be able to post photographs of the right bolt lug and the firing pin (specifically the tang and the firing nipple)??
Brobee
edited to add: It would also be good to ask if the round was loaded by putting the round into the chamber and then letting the bolt fly home. If there is ever a probability of someone loading the gun this way then I agree that CCI#34 NATO Spec Large Rifle Primers, if not then I don't think they are necessary and have many many many thousands of rounds of CCI Standard Large Rifle, Winchester Large Rifle, and CCI Large Rifle Benchrest though my many M14 type rifles with no incidend (other than high primer scares).