I have a 300 ultra mag that I believe is due for a new barrel. I struggled with this question when I first purchased my 300. 300 or 338 Lapua. Now that I had decided on the 300 back then, some of my perspectives have changed a bit, but the question is no less easily decided. This will be an elk rifle looking to reach 1000 yards if necessary, but want still to be as close as I can. 338 Lapua is out because I want to rebarrel, not reface bolt. 338 Rem ultra has the same bolt face, so no need to open the bolt face. Action needs no work. I have a lot of 300 RUM brass and bullets.....this also plays somewhat into my decision. Does anyone know if you can take 300 brass and resize to 338 ultra? This knowledge will also help in my decision. Any opinions on caliber choice are also welcome. Thanks in advance!