Can a case be stretched by using a full size die on it?
The stretching/separation happens on firing ammo that has a loose fit in the chamber. You should not have had this with setting the rifles headspace on a factory round, then neck sizing. Very strange, has me confused. Savage owners have been screwing on after market barrels & headspacing using factory loaded rounds. At times, brass that is FL sized, will not fit into the chamber of the rifle. This is because the rifles headspace is set to short & the sized brass has "stretched" The factory ammo will be on the short side when measured from head to datum. The FL die may "stretch" this measurement longer, as the case body is sized. Just like when the trim length gets longer when FL sizing.