Out to 600 you probably don't need magnification into the 20x range, unless you're trying to spot hits on paper or something? I'd recommend a decent 4x or 5x zoom ratio scope with magnification into the teens. Vortex PST Gen 2, Nightforce SHV or SHV F1, Bushnell LRTS, etc.
I had a 1-6x on a .308 carbine and I too didn't like it much past 300 yards.