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.
