I would measure the diameter of the expanded neck of one of the fired cases that didn't split. Compare that to the neck diameter of a loaded factory round from the same box. The difference should run at least .005 and not more than about .008. It it is bigger than, say .010, the neck of the chamber is cut too big - and the necks are swelling too much. If not, it is probably a brittle batch of brass. Has this gun been shot much in the past with factory ammunition? Do the loads show obvious pressure signs - like cratered primers, extractor mark, or hard bolt lift?