I am having a rifle rebarreled to a 308 Norma Magnum.I got my Redding die set in so I played around with some brass I have on hand.I necked up 7mag brass and it was easy with no problems.I necked down 338 Win Mag,it was easy with no problems.I resized 300 Win Mag brass and I think this is by far the most difficult route to go.It totally reshapes the shoulder of the brass and requires trimming afterwards.I don't mind the necks being short using 7mag and 338 Win Mag brass,that has never been an issue for me since I make 338-06 brass from 30-06 brass and that also gives me a little shorter case.The main thing is brass should be new,once fired or annealed and it makes it much easier when the brass is soft.There is also the option of purchasing Norma brass if you can find it.