SGJones got it right there. I doubt the actual shell had much to do with this burst. Doesn't really look like an obstruction burst either. Possibly a "Seamy" piece of steel that finally just took all it could & turned loose. In spite of all the condemnations of damascus a flaw such as this is as apt to show up in fluid steel which is "Drawn" into bars. A flaw is thus stretched into a seam which can be totally hidden in the wall.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra