300 Winchester Short Magnum
I'd suggest getting one box each of the 180's Hornday Interlock, Horady SST, and Nosler Accubonds and see what works best in your rifle.
If you find a load that it really likes at 300, then move on out to 600. If you can maintain sub MOA at those ranges you have a load that will work fine.
It's going to take a good bit of practice and good note taking as to conditions and results to become proficient though at ranges beyond 600. Out that far it's just about marksmanship, beyond 600yds though life gets pretty complicated.