A small group of close buddies and I hunt a public land drop camp in Colorado every year, we have killed a lot of bulls. 75 percent of them killed 350-650 yards. I am by no means proud of shooting this far and myself really try to keep it around 500 max. The problem is on public land with extremely pressured elk, vertical terrain, 20 foot tall scrub oak, cross canyon stands, the animals only come out for a small 30 min window in am/pm and there is a very small chance you would be able to cross one of these enormous canyons and get closer and ever see the animal again.
We shoot 300 mags of one kind or another with quality bullets, I generally shoot 175 LRX and my buddy’s accubond, both have had great results.
Something like a Leupold VX5/VX6 3-18 WITH and zero stop lock. Then dial it in really good, get all your calculations triple checked and test your custom turret thoroughly when you get it, practice like crazy and also understand your altitude differences, I live at sea level and we hunt at 8,000 feet, there is a bit of difference there. Practice more.
Rifle: I love my 300 wsm model 70 crf. I prefer something that feels really solid to lay down over a backpack and shoot 500 yards but does not have the heavy target rifle feel, I still want it to be trim enough to pull up and shoot standing/kneeling at 50-100 yards. I traded the factory trigger for a 2.5 Timny. 7.5 pounds scoped is probably ideal rifle weight. In vertical terrain and packing out meat, your back pack and rifle just get so f***ing heavy.
If you want to go classic, I would not look any further that a new model 70 featherweight in 300 wsm if your more modern feeling go with something like a Christensen, glass bed either one.
One other thought, we all use muzzle breaks but we have all learned that the first thing we do when we spot and elk is to start fumbling around and try and stuff some kind of ear plugs into our ears. This becomes a huge pain, idk how I feel about this. Oh and practice some more, off a backpack and some kind of sticks too…
Good luck!