Read up on the "Cream of Wheat" aka "COW" method of blowing out a smaller neck case to fit. I had read on this site a guy using 300 Win Mag cases, 14 grains Unique and fill case with COW - no bullet - fire it in 458 Win Mag chamber and that blows out to close enough to re-size for 458 Win Mag - has to be trimmed to correct length. I am doing the same with 7mm Rem Mag brass, but I annealed the necks and shoulder area first - not sure if that step really needed, if prepared to loose a few?? To my mind, an "inexpensive" source of 458 Win Mag brass for reduced cast loads - not sure I would use them for "full-house" 500 grain plus jacketed bullets?