"Headspacing off a case shoulder may leave you with a unsupported area in front of the case belt in the web area and thus allow a catistophic failure. "
That's hilarious! You are joking, right?
MSLR, if a cartridge chambers snugly, belted, rimmed or neither, it's all as well supported as is possible to obtain and there is MUCH less hazard of a head seperation too. We aren't resizing to match the steel headspace gages gunsmiths use to cut a chamber, we are loading our own brass cases. For some 40+ years, I and every magnum reloader I know of has FL sized thousands of cases to fit the shoulders to the chamber. Guess how many "catistrophic failure" events we've experienced!!!