My choices in caliber would be 338 wm, 9.3x74r, 416 remington. I picked these simply because that's what I have. I have owned a 375 H&H mauser that I loved and I think it wold have been great on brown bear, but it was too nice for those conditions. I've also got a great 9.3x74r Chapuis double that would work well as well, but again, I don't know that I would want to risk ruining that gun prior to its trip to Africa. I've got a ruger 338 winny and a remington 416 remy, which would be my final choice. The 416's shoot flat and accurately enough for those ranges mentioned, and I don't think one can have too much gun when hunting any type of DG. Also, bigger cals equal bigger, bloodier holes. Moreover, these all equate to fewer injured and more easily found animals. A tough, but fun decision.