This 'catastrophic failure' of a Norwegian Krag occurred in Great Britain at a practice match in the Spring of 2014. It was investigated by the NRA-UK. The 23 page report was posted today on the Military Surplus Collectors Forum (under Krag rifles).

The Norwegian Krag action (#32513) was of 1897 Steyr vintage. A Norwegian match barrel was installed circa 1950 and the rifle has been in routine use and well maintained. Nicks and alterations to the action receiver ring and threads led to suspicions regarding the installer's competency.

The 'home loads' (reloads) used Norma brass, 140 grain bullet, and 34.4 grains of Accurate 2520. (This load appears just above a 'starting load' and well below Max. in one Accurate Arms table, I found). The rifle came apart on the fourth shot. The 'shooter', a neighboring 'shooter', and a spectator were injured and required hospital treatment (surgery for two and OP for the third).

The NRA-UK report noted 'grossly deformed necks' on some of the 'homeloads', which indicated improper setting of equipment. They displayed one of the fired cases, which looked normal. However, IMHO, the photo of a deformed loaded case in a Wilson gauge, appears to show a possible head-space problem, caused by excessive case length or poor adjustments. (I wish the report gave more attention to the dimensions of the fired cases and case length of the deformed necks).

Some parts of the destroyed Krag-Jogensen could not be found. The NRA-UK found no clear definite cause and called it: "A single overload event". If I understand the report's conclusion correctly, they attributed the failure to an excessive load or latent defect in the re-barreled rifle.

(The excessive pressure could have been an under-load detonation, an excessive pressure over-load, or something related to the 'funny' case necks).

Detonation is a very plausible cause here. Fast powders and low case density are a dangerous thing in rifle calibers.  Too large a diameter of projectile could also cause this.

A note about the Brits.  There's a story going around the FAA about the Bits.  They were looking for  a way to test the windshields for their high speed trains.  They asked the FAA to borrow the "Chicken Gun" that we use to test aircraft windshields.  It actually shoots chicken carcasses at the windshield. Well, the Brits, using the Chicken Gun, busted just about every windshield the had and sent a report to the FAA for review.  They couldn't understand why all their windshields were breaking.  After the FAA read the report, they sent the Brits one recommendation.  "Thaw out the chickens before you shoot them".

Yeh, one forward lug, a bolt guide rib (fitted to make contact with the receiver, unlike U.S. Krag bolts), and the bolt handle root.

I guess my point was: The bolt was not the weakest link.

It appears the fellow needs some reloading lessons.  SEE is a possibility.  Low charge, long and perhaps rough throat.  The bullet stops for a millisecond, the powder keeps burning and pressure goes through the roof.  6.5 Swedish is notorious for this, not just in Krags, guys have blown up more than one Swedish Mauser with loads that were meant to take it easy on the old gun.

I'm not an Engineer or Metallurgist, but IMHO, this mischief was caused by extreme high pressure gases (due to over-load, under-load detonation, bullet/throat problem, compromised cartridge case???) blowing out the case head and allowing the gases to enter the action. (Metal fatigue or flaws may have contributed, but, probably not necessary or root cause).

When rimless cartridges and their rifle actions 'blow up', they look kind of similar, with top of receiver ring and extractor gone, and other wreckage somewhat restrained by wood.

At risk of appearing totally out to lunch, what is SEE?

Thanks, BR.  My burn rate list says Accurate 2520 is just a smidge slower than IMR-4064 and a good bit faster than IMR-4320.  Jacketed bullet loads in the 34 to 36 grain load range have been in use since the 1930s for US Krags and the '06.  My guess is that something else was amiss.  Those case necks did not look kosher.  Bullets for a Carcano?  IIRC, those run .266-.268 and come in boxes that say 6.5 mm.

That Lee Navy makes me want to cry.  I spent the better part of 3 years trying to find a good one at a price I could afford.  I quit looking when those accidents appeared on the net.

Butlersrangers, that Swiss blowup kind of messes with my theory. It would be interesting to see a high speed video of each of the blow ups. After the high pressure event they are not coming apart as I would have expected.
Just a couple of notes on the US Krag I just rebarreled:
The bolt did have a lot of over travel with no case in the chamber. If this is the same with a 6.5X55, the bolt could follow a severely resized case way to far.
The chamber was .010 to .012 larger than an unfired case.  Not sure if this was from an oversize reamer when the gun was first assembled or if the barrel swelled with use over the last century. If it swelled with the relatively low pressure loads then the  barrel steel is fairly soft.