I prefer fireforming without a bullet, in cases where the donor case is already close to the final length. My previous experience, with the common expander button in common loading dies, shows that significant trimming to "square up" the neck is often required. Expanding a .243 neck to .257 will already shorten a little bit, so unnecessary additional trimming should be avoided. I think I would try sizing a 308 in a 250-3000 die until it just chambers and fireform it w/o bullet. then if the neck in too thick, I would decide whether turning the necks or switching to 260 is better overall. I have plenty 308 brass, but not much 260 or 243. I don't have much trouble with thick necks unless I start with donor cases (like 30-06) that have to be shortened well into the body area. I suspect 250-3000/250 Savage dies can be used to load the .25 Super. They should only need to be carefully adjusted (after smoking the case neck/shoulder) to not set the shoulder back. Buying a new set of 250 dies would likely be cheaper than having custom 25 super dies made, if you don't already have them.
Mike
