Re: ideal caliber
I agree with Ric on the 300 Win mag for the ranges mentioned from 600 to 1000 yds.
Excellent brass can be bought for reloading and in a pinch, good factory ammo is available.
If going to 1000 yds, the 220 gr MK will outperform the 190 or 200 gr MK.
At 3000 FPS (WITH A 28 TO 30" BARREL) the 220 gr will still be going 1769 FPS and the Foot ponds of Energy will be 1528 at 1000 yds.
If you want less recoil you can go down to the 190 gr. If you have a muzzle brake, it won't make a differance.
300 Win for the ranges mentioned would be my choice.