My gunsmith couldn't remember, it's been a while since mine was off and rechambered but Ruger would know for sure. I was curious myself but just haven't called them yet. I'm not sure but the Ruger "Magnum" is up to the 416 Rigby and the "Express" can't be had in the Rigby size though. I think the largest in the Express is the 338 WM or 375 H&H. There maybe a differance in the actions thread length and diameter too. Another question for Ruger.