Get closer to this buck. Start working your way to longer ranges in the off season. Now is not the time to "learn" to shoot 600 yards. The guys that shoot long have been doing it a long time. I'm at 500 currently for my limit, and there's been a lot of time invested to get to that range. If you know where this deer is coming from and going to, set up closer. You don't gotta do it from a stand. They are just as dead when you shoot them from the ground. Good luck!
-Jake

Night Crawler .... The typical 270 Win. hunting round using a 130 gr. bullet (which is a great choice for medium game) does about 3100 fps at the muzzle. Zeroed at 200 yards typical drop at 400 yards is about 17" and about 36" at 500 yards.

A typical 308 Win. hunting round using a 165 gr. bullet (which is a popular hunting round) does about 2750 +/- at the muzzle. Zeroed at 200 yards the drop at 400 yards is about 24" and 500 yards is about 48". Even if you back up the 308 Win. bullet weight to 130 gr. you do not gain enough muzzle velocity to make that much difference, besides the down range lethal performance of the 165 gr. bullet out of a 308 Win. has been long proven.

Either round is deadly on deer at thiese ranges. It is just a matter of of managing the drop.