Like someone mentioned before, you just have to figure out what game your gonna be shooting..
Personally I don't consider 400-500yrds "longrange shooting"
Most standard cals are capable of hitting targets at that distance time and time again have more than enough energy to dump deer size game out to 500yrds..
Moose aren't that hard to bring down with a 30-06 at that range either...
However things change when your shooting Elk...
If ya want a 500ryrd gun capable of shooting deer moose elk sheep goat
Personally I find the 30cals group better at longer distances, but heres some that could work for ya.
270WSM any of the 7mmmags and 300mags 325wsm and .338win mag...
If ya can handle the recoil of anything bigger than the .338 give em a whirl...