Wow! This is an interesting incident. I noticed a few things from the pics.
1) The remaining chamber appears to be oval in shape, rather than round. Could be an illusion due to the fracture itself mot being perpendicular to the chamber.
2) There is a discoloration "reddish/brown" on about a 1/4 of the failure face and five or six darker "pits". I wonder if the metal itself had a deficiency or had already yielded due to torque or an excessive point (compressive stress) load resulting in a fatal weak point the the cross section of the chamber. The other thing I noticed is that the darkened area was on the underside of the rifle, the area where plenty of cosmoline would have been during shipping. It could be the darkened area was cosmoline which had seeped into the fractured area. No gas would have leaked out because the chamber was nice and tight (good spec!). Then again, could have been residual dirt from the barrel landing after it's little trip.
3) The casing shows the bulge, but a good portion of the neck is seemingly perfect in terms of round. It's almost like the the force of the exploding powder, the resistance of friction of the barrel against the bullet and the fractured chamber propelled the barrel like a rocket. A squib would have ruptured the barrel into ribbons.
4) Most telling was that the bolt remained in battery. That suggests the barrel was already in travel prior to the bullet passing over the gas system. The pressure of the powder building would have been partially absorbed in the expanding case and open to atmosphere by the moving barrel likely before the bullet was 1/4 of the way down the barrel. The remaining gas and momentum of the already (grooved) bullet would have been sufficient to allow it to exit the barrel.
My thoughts only.