I think it can be reliably stated that no one would really consider going after big game at 400 yards with an AR- 15, but I think the AR-10 in .308 will reach out to 1400 yards.
I ran with a .308 savage back in the day... Figured it was a good all-around gun. And it was. but it didn't take long before I realized I needed a faster action in the saddle and in the bush. So I bought my 30/30 Winchester and began to carry that, consigning the .308 to hunting blinds and such.
But then I went after elk. I don't remember if I ever blooded that .308 with an elk, because I went pretty quickly to a 30.06 with a scope. But I was still beating through miles of tangled brush to try and get a shot. It sucked so bad, and half the time I was in too close and spooked em... Quick shot, nothing but hair in the scope.
It wasn't till my first Bighorn that I figured it out... now I use big iron, with good optics. I find a ravine the elk travel to get to pasture or water, and I find high ground overlooking that travel, and I wait.
No more brush beating, no more uncontrolled shots _ I take my pick now. It's always a fairly long shot that way... say 400 yards and up... 600-800 is alright. 1000 yards is a stretch.
All that to say, if you're getting 1400 yards out of a .308, you must be dunkin them rounds in secret sauce.