One reason a 308 is so highly regarded for long range work is that its light recoil makes it easy to practice enough to become a good shot way out there. And practice you must to hit past 300m, much less at the kind of ranges you mentioned.
I would also brush up on tracking, since your 30-30 analogy is so accurate. Few consider it a 200m deer gun, and most would expect a long tracking job on a deer shot at that range with one. Perhaps the biggest problem is simply finding where the deer was standing when the bullet struck. A third of a mile is a long way in the field, and things look different there than they do from your firing point. In such a case, I would mark my firing point by hanging a marker such as an extra orange vest in a tree. (A flag made of surveyor's take will be too small to see at that distance.) Shoot an azimuth to the game's last known location. Walk downrange, shoot a back azimuth to the marker and use your rangefinder to position yourself along that azimuth. Then you can start looking for spoor.
I used a 308 a fair amount in the military and made some interesting groups with it at the ranges you talk about. One reason I could do this is that our doctrine called for two-man sniper teams, with the less-experienced man on the rifle. The seasoned guy ran the spotting scope, read the wind and told the shooter where to hold. Among guys who knew the deal, any glory that acompanied a long shot went to the spotter first, the team second and the shooter last.
So the best advice I can give you, aside from dropping this project altogether, is learn to read the wind. There are mechanical ways to deal with trajectory but reading wind is still an art.
Okie John
