I make 8 X 57 out of .270 cases. One of the keys to this kind of case forming is to aneal the brass before working it. Anealing the necks will make the process easier and result in fewer cracked cases. I can neck .243 up to .308 and any 270/280/30-06 family of cases to 8mm w/o problems. Failing to aneal will cause premature cracks in the neck. Depending on how you work the brass, you may get more than 2-3 loadings. The case separation is partially caused by stretching. You can minimize that by lubibng the inside of the case neck prior to sizing. I use resizing lube, but you can use mica if you choose.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball