VERRRRrrrry Interesting.
Diagnosing from the pictures posted,
without more pertinent information,
will have to be mostly speculation,
however,
to me,
it looks like that bolt WAS in battery when the failure occurred.
1.] Pics 1,2, & 3 show us that The extractor is still there. Since the extractor is only supported by the barrel hood for the last 1/8" or so of bolt forward travel, if the ruptured case exploded before the bolt was almost completely closed, then the extractor would be some where else, not still in the bolt.
2.] in pic 3 we can see that the bolt lugs are fully engaged. The crack shown from the extractor hole to the left lug shows that the bolt lugs were engaged, and did the job of holding the bolt in place in the receiver, at the moment the traumatic experience occurred. Again this is an indication that the bolt WAS in battery when the explosion occurred. If the bolt was NOT in battery, then the lugs would not have been engaged, and they would have stayed intact, with the ENTIRE bolt flying backwards.
3.] The bottom view in pic3 shows us exposed ejector spring, and a clear view of the left lug and that small angled surface. Look in the bottom of your M14 type rifle .... there is supposed to be a chunk of steel on the receiver side wall that masks the left lug from view. That chunk of steel is gone ... peeled off the side of the receiver. The exposed extractor spring indicates that high pressure gases from a case head rupture or pierced primer forced their way into the ejector spring hole, and peeled the bottom of the bolt off.
So, without further inspection of the fired case, and the stripped bolt, and more information from the shooter,
I am going to suggest that this explosion occurred WITH the bolt fully forward.
Most likely culprits:
1.] MASSIVE overpressure
due to bore obstruction, bullet setback, poor reloading, etc.
2.] Case head separation
due to case stretch and poor reloading, and/or above
3.] Pierced primer due to firing pin tip broken into a sharp point, or too much extrusion, or brass shavings in the firing pin hole [ NOTE this is not quite at likely as the path of the gases and the ruptured stock suggest the ruptured gas came out the side, not the primer hole ....
UNLESS ,
the second round in the mag ALSO EXPLODED???]
4.] Bolt that was too hard and brittle
5.] While we are speculating on insufficient data ....
what happened to the rest of the loaded magazine???
Did the next round in the mag also detonate??
I don't know about you guys,
but I'm going to take my Norc M14 rifle to the range today
and SHOOT IT WITH JUST AS MUCH CONFIDENCE AS EVER.
We can blow up ANY gun if we try hard enough,
ANY GUN!!!
and we can be hit by lightning while playing golf too.
I'd say the odds of either happening to me today are less than me winning the lottery [ especially the golf, because while I do buy lottery tickets, I think a golf course is a waste of good real estate that could have been a range ].
Which reminds me ....
I better check my lottery ticket,
AND,
inspect the firing pin hole of my M14 bolt for brass shavings BEFORE I pull the trigger,
AND
check my tires before I jump on my FJR 1300 and tear off at 100 MPH
AND
....
you get the idea.
[;{)
LAZ 1