Re: 270 wsm split neck
One other reason might be headspace. You mentioned that at first the rounds would not chamber. I don't know if your overall length was too long or maybe it was headspace. If you full length size a case and it is hard to close the bolt then it may be your headspacing. Turn the sizing down about an 1/8 of a turn or so at a time until the bolt closes easily - like factory ammo. I had a few 270 wsm cases split on the necks like you described due to this. After I fixed that I now get about 5 reloads on average before the necks split. This is without annealing.
Just another thing to consider