factory rifle in 300 Win Mag that will allow him to shoot Elk at around 600 yrds. and he wants the rifle to weight in at around 8.5lbs. with scope.
I guess we could consider this to be an experiment in 'what if'.
Most of these 'requests' come from folks who lack the most basic understanding of firearms and ammunition with regard to accuracy over long distances. They make the request for a lighter than normal rifle which will perform like a premium built tactical rifle because they don't want to have to carry the extra weight.
Too often there are discussions on several 'long range hunting' websites lauding someone's adventure with successful shots at extended ranges. This can have the effect of giving false confidence to shooters who don't necessarily have the ability to accomplish that same event but believe they do.
For the sake of discussion though, it's possible to examine the possibility for a potential candidate if we ignore the costs. This is just for the sake of discussion in the spirit of examining what is available, not what the shooter is capable of. I do not support any contention that long range shots should be made without regard for the game.
Browning introduced the Hell's Canyon Long Range McMillan featuring the X-Bolt action, designed specifically for long range accuracy and hunting. It has a 26" barrel, chambered for the .300 Win. Mag. and weighing in at 7.5lbs without the scope an mounts.
Couple this with the highly regarded ABM ammunition for hunting using the 168 Grain Match Hunting bullet from Berger. At normal hunting velocities it should have just about 1600 ft/lbs of energy at 600 yards.
I think you could also consider either a Sako or even the Tikka for such a project given all the accolades they garner from many shooters.
Now the trick is finding a GREAT scope specifically for precise shooting at long ranges but in a lighter weight conformation. I'll leave that part for the folks who are more attuned to the lighter scopes and mounts.