There are 2 parts to the question the first part is irrelivent the most inherent accurate cartrige is the 6mm PPC but useless for any application where energy levels are an issue at 800 yards. the seccond thing what do you want to shoot" what energy is required that is the most important selection before we look at what rifle and cartrige combination as we all like clean dispatches of game.
Any well built rifle should get you groups under 3" at 500 yards with practice not every group but when the conditions are right and the shooter has the brain turned on. With this in mind i would look at 6.5 and 7mm's unless the game is verry large where a 30 or 338 could be an option. now as a factory rifle a 260 rem on a Rem 700 VLS this gives you an option to rechamber it to 6.5-284 or 6.5x55 improved to give you verry good performance. Now if there is not enough energy i would look around for a Rem Sendaro SF in 7mm Rem SAUM these rifles are used in 1000 yard benchrest in Factory class and are verry capable being able to stabalise 175 and 180gr match bullets and push them just under 3000 fps they are hard to get because i dont think they are made in a heavy barrel any more. i have a 7mm SAUM on a model7SS and it shoots 3/8" groups with a Limb saver Barrel Deresonator fitted and the trigger adjusted with no other changes the case is perfect for a 7mm Bore and i believe if you require energy at 1000 yards it is a great option you can also have a custom rifle
built. The 7mm WSM
is ok aswell but i dont like Win Actions and prefer the Rem short mag in 7mm.
what do you need the energy for??? as i may not have given you enopugh gun but it might be perfect.