J-B Welder, I can explain the failure rather easily. this was a failure of your brass. it was brittle and/or fatigued. this failure is called a "dog bone split". it is tell-tail of brittle brass or over worked brass. this happens to brass once the back half of it has been sized too many time and it just will not be elastic enough to take the pressure. do not believe this is your gun's chamber. I have seen this a great many times even before I was a gunsmith and saw this professionally. I have this happen to me a lot on used brass I get in and sell out of my shop. someone brings in their husband's/dad's old brass and there is a few pieces that have more than 7 reloadings on them and they just do not survive much longer. I have had it frequently happen on my test fire factory loads over 30 years old.