I would say if you have never owned a bigger magnum start with the 300 win or the wsm cause the recoil is alot more on the ultra mag When I first got my 300rum I couldn't beilevethe difference. And when I used the 300 rum for deer and you get a little to close to the shoulder their is alot of meat loss.As far as the 800 yard shot thats along ways to be trying to shoot and kill something efficently not saying you couldnt dont know your experiance but if you can go for it go to www.longrangehunting.com they will help you with that advice. I think all the calibers mentioned will shoot that far cause I was reading a post were a guy shot a elk at 946 yards with the 300 win mag. I wouldnt try that shot to many variables and i like to actualy hunt and stalk not just shoot a half a mile