The case which separated was an older case and not a once fired.
This would seem to be your problem, brass of an unknow history. The load is ok. Mag. primer is good for that powder. Try sizing your new brass with .005" between the shell holder and FL sizing die. See if sized brass will chamber. If it does, this will help the brass last longer. Check this each time you load for at least 3 loading, as the brass may not fit the chamber after it been reloaded a few times. http://www.saami.org/PubResources/CC...0Remington.pdf
The OAL seems long?? But if enough bullet base is in the case, not a problem.