I have read stories (in the past) where guys were pushing 22LR out to 500 yards on large-ish gongs. I even saw a video of a guy making hits. Granted, you have time to have a sip of coffee between the shot...and the hit.
If memory serves, that particular fellow had an adjustable (MOA) rail. Might have been this one;
http://bulletin.accurateshooter.com/...ro-to-150-moa/
Seems a bit extreme maybe...but if you want to really reach out an have a little flexibility in the scope choice because of elevation limitations....