My vote is for an 18" or 20" AR upper with an accurate SS barrel (WOA, Lothar Walther, Black Hole Weaponry, etc.). I was in
AZ last month with my 18" SPR, smoking prairie dogs out past 350 yards using 55gr Sierra BlitzKing loads from Australian Outback ammo. I shot from the prone off my bipod, off shooting sticks from the kneeling, and from an Idleback chair.
If you want to go even further with the .223 loads, I'd look into some of the heavier projectiles, which will have a better BC than the 55gr stuff. My furthest kill of that trip was 372 yards, and the 55 grainer still about blew the dog apart. However, it's windy out in desert and grass lands of Northern AZ, so that played a factory often; and caused me to miss several shots at 400+ yards. A heavier projectile with better wind resistance would have helped me connect on the longer shots.