Jimbob, I can't imagine anybody shooting at those ranges in an area he is not totally familiar with without a good rangefinder.
I was responding to tahunua's assertion:
if you know your bullet drop, wind drift, and FTLBs, have a good idea of the range and can make a good determination of where that bullet needs to go then it is not unethical to move from a 200 yard max to a 500 yard max.
The example also illustrates many of the other variables invloved, as well.
Wind and time of flight are big ones.
Wind can only be read with practice.
The biggest factor is the shooter being able to shoot in the conditions presented.
Were it me, I'd want a flatter shooting cartridge than the .308 WIN if I were going to try to shoot that far.