The flattest shooting calibre? Kwk 42 L/70 with a Pzgr 40/42.
73,000 grain projectile with a muzzle velocity of 3700 fps. The BC is off the charts! Just fab up a mount and weld it to a sturdy trailer, BOOOM!!!!
Now if you have the coin and connections you could skip the old milsurp and get the ultra fast M256 120mm (mv 5700fps), but that thing is a tad excessive for north american game and 4000 meter shots are unethical imho.