I think it is doable, just not as easily as with a bolt rifle. I think that is why scope makers that make scopes specific to the AR platform and the .308 round limit their BDC reticules to 800 yards. There is enough room for hold over to go further and figure MOA to be about 10" at 1,000 yards. So if you can hit within about 10" on a 24" or 36" target, you are doing very well.

I say go for it! Hell, if I can just hit the 24" target at that range I would be happy. That's a lot of distance and weather influence to deal with.