I would suggest 300 Win Mag. Ryan Cleckner carried one in service in the GWOT when he as sniper team leader in the 1st Ranger Battalion 75th Ranger Regiment. His job was contacting an IPSC kind of target at various distances. He would dial elevation and hold left or right edge of target zone into the wind. Granted, most of his shots in service were less than 600 yards but it is a good place to start.