Shot placement is the first requirement.Very flat trajectories are helpful to a point.
Some of the load manuals offer "maximum point blank" ranges for given bullets/velocities,this is how far you can shoot with + or - 5 in trajectory.
I'm not going to go get the book,but I don't thnk any 7mm mag/300 win mag loads will make 400 yds with max point blank.
1970s,80's,about the best rangefinders were a cheap plastic split image rangefinder that did not work all that well,and a reticle like the mil-dot.Mil dots were uncommon and expensive.A few of us figured out the mildot rangefinding principle could be applied to the known size of a duplex.It was useful that a 6x by 42mm leupold duplex was one mil,center to post.
With those tools to estimate range,and a knowledge of our trajectory,we could "kentucky" and reach out a ways.
Now,folks have 1500 yd lazer rangefinders,kestrel wind meters,I-pads with nightforce software,etc.and great scopes with target knobs
The flat trajectory factor is less important.
You need enough velocity at the animal to make your bullet perform.Bullet manufacturers provide that info.A 165 gr .308 Ballistic tip,I will guess,needs about 1900 fps.That will probably limit your extreme range
You can do all that.
Myself,I use my .257 Ackley on a Mexican Mauser with a 6x scope.
I can rangefind with my duplex well enough to know how to hold,or "too far,get closer"(If an antelope,brisket to whithers,fits right in one mil,its some over 400 yds,I get closer.If he's bigger than one mil,I can look at setting up a shot.)Better he is 1 and 1/4 mil,or 1 1/2.Thats getting closer to 300.
No fumbling with rangefinders,no extra motion,traveling light,and effective.
And no problem keeping my eyes open as I squeeze the trigger.