Why an AR if it’s “mostly for long range? Yes it cam be done by the way, but it’s not near as simple as you may think.
Do you reload?
If no then 6.5 Creedmoor
If yes then 6mm whatever, 6.5 C, 260 Remington, 7-08.
While a .308 will shoot a 1000 yards it’s not ideal, and with the speed...