Did YOU swap barrels, or did a smith? If a smith did the job, he should have set the Improved head space so there is a slight crush fit with virgin 280 brass when you close the bolt. No need to neck up/down, and really no need to use the COW process, just load a full power 280 charge with the 140 grn. and shoot it. Cases will come out perfectly formed.
I`ve a number of "Improved" cartridges/rifles. Have not had any problems with the above procedure.
THIS--but only if it is new brass. Fired brass tends to lose the crush-fit "curve" at the neck/shoulder juncture, so may or may not go bang.
There's no reason to spend time, powder and primers using the COW method on a big game rifle: Just load maximum .280 charges for the bullet and go to the range--or even hunting. (That said, I have used the COW method considerably when forming is more complex, as when both necking down and blowing out brass. But I do NOT use paraffin, for the reasons already suggested by others. Instead I just stuff a bit of crumpled paper towel in the case neck and fire-form with the rifle held muzzle-up. This is quiet enough to do this in my garage, and none of the neighbors has complained yet.)
