Check your sizing die settings. could be a situation of excess headspace- by allowing too much headspace the brass is stretched too much at firing and then splits, this could be your problem. try re-firing the cases that didn't split, necksize only- when reloading- and compare the number of bad cases per hundred of the reloads with the original mortality rate. If it is significantly better, I would bet you had too much head space.-size your cases only enough that the gun closes and locks up. To neck size only, color the neck of the case with a magic marker, or "smoke" the case neck and adjust the sizing die down the neck only far enough to grasp the bullet, I go about thre quarters of the length of the neck- the sizing die will scrape off the ink/smoke as it progresses down the die.
