Not to pick at the OP particularly, but this question comes up so often I don't understand it. First, why does someone in the market for a new rifle want to let someone on the internet he hasn't even met pick out a new rifle for him? No personal preferences of his own? No loyalties to any brand, cartridge, style, etc....
IMO, if you want to shoot 600 yards for game (with a 1000 yard rifle, whatever that is defined as), I would think you would do some homework first. A 300 WM sounds like a great place to start if you don't know anything about ballistics, bullets, scope adjustments, etc.... but how about seeing what rifles really work at that range with the available bullets? And, if you do find a 300 WM load that is accurate enough and has a decent BC to shoot past 600 yards into a game sized target, can you afford enough of the ammo to shoot enough to become proficient to shoot game at those ranges? And if you can, will you develop a flinch or other issue during your "training period". This is a serious question- I've seen it happen and in this day when premium ammo comes pretty dearly at $60-$80 a box , it can get pretty expensive to reach proficiency with your chosen weapon.

Better choice is to do some homework to see what actually works before plunking down your money. Stick and I have had this discussion for as long as we have both been on this forum, which has been from the beginning. However, he actually has the perfect place to experiment and the willingness to do so and knows what works and what doesn't. A 308 caliber bullet with a lousy BC isn't going to go to 600 (or any other long range for that matter) any better than a . 6.5 Caliber or .284 (7MM) caliber with a huge BC and have any more energy or ballistics. And, the shorter cases will be easier to shoot, cheaper to learn on, and have more choices for rifles these days in styles and weights you want to carry into the hinterlands. The 6.5 cartridges aren't just a craze that will die out tomorrow because guys have finally caught on to the fact that bullets with astonomical BC's in the . 650 and above range are flatter and work as well way out there than their fatter cousins in many cases.

To make a long story longer, do your homework and open up your horizons and you may (or may not) decide on a completely different tool. But at least it will be YOUR choice, not some incoherent voice on the internet that may or may not hunt in the same conditions you do...

Bob

Never underestimate your ability to overestimate your ability.