Start with .280 Rem brass and the shoulder will be fully formed when it comes out of the forming die. Neck to .25 cal as an intermediate step.
I'm hesitant to go that route. In my zeal to form .257 Wby cases from 7mm Rem Mag brass, I got into accuracy inconsistency problems, to the degree I abandoned the .257 Wby. and rebarreled my Mark V to a match chambered 7mm Rem Mag, #2 Brux at 24".
Safariman ended up with the take off .257 Wby. bbl. On relating this story, Mark said he had been there, done that. He told me that 7mm Rem Mag brass formed into .257 Wby. brass has excessive neck wall thickness and if the necks aren't turned, inconsistent accuracy results. He said that problem is eliminated by using .264 Win Mag brass. I turn 6.5 and .224 necks but am not looking for more neck turning if I can avoid it.
So, given that, I think the less forming, the better. Using .25-06 vs. .280 parent cases forming the .240 Wby. may well be similar to using .264 WM vs. 7mm RM, forming the .257 Wby. .240 neck wall thickness looks OK with brass formed from .25-06.
YMMV, let me know.
DF
