Looking at both fotos of the blown up guns I am wondering if the threaded ring did not fail first. Maybe the relatively soft barrel swelled splitting the ring and as the barrel moved forward after the ring split, the extractor pulled the rim till the rim/case failed. The American Krag is not particularly weak in the area of the cut. I just finished rebarreling an action and I was impressed with the strength in that area. The Brits may have been on to something when they mentioned the marks on the receiver. A pipe wrench applied there during rebarreling would make deep marks that would concentrate stress and drastically weaken it.