Lots of cals will do the job with more than enough energy to kill these animals at these ranges...
And its not as hard as some as these guys are saying..
Time dedicated to practicing is the number one factor.....
If you can't put aside this to do some serious shooting,it doesn't matter if you have the best of the best stuff.....
The number 2 factor common sense....
Alot of hunters think just because a few of us practice long range shooting at targets we have no problem ripping a shot off at game any time we see something at that distance....Nothing can be further from the truth..
Serious long range shooters and long range hunters know exactly how far that target,animal is and know where there bullet will hit..
Serious long range hunters know exactly when to fire and know when to hold off....
I personally can count on three fingers how many times I've killed animals between 450 and 520yrds....
I have not killed anything beyond that distance of 520yrds however I've spotted "hundreds" of game animals that never presented a quality shot between 500 -650yrds..
Because I live out west, and hunt alot of alpine and sagebrush, most of the animals I spot arebeyond 100 to 125yrds that most hunters shoot them at..
Most game I spot is outwards of 200yrds, and shoot game regualarly at 300yrds.....
The next is quality optics and invest in a steady rest..
My advice stick with a cal your able to handle shooting lots,
find a powder bullet combo that will kill efficiently at that range,and one that you can make tight tight groups with consistantly.
I myself find that the .30 cals group tighter at longer ranges
than alot of the .7mm....
The .270 cal in any of the win, wsm and weatherby can handle shooting and killing at that distance as well, but I would replace any of themfor the 6.5 x.284 when it comes to flat shooting inherent accuracy..