The trick with the 375 and 408 is that they use special projectiles that do not start to loose accuracy during the transition from super- to subsonic (which results in shock waves), which totally wrecks accuracy for most projectiles. Except those that never break the sound barrier to begin with, they have been shooting and hitting targets with the .45-70's 140 years ago, with black powder loads out of single shot Sharps, at more than 2 kilometers. Very slow projectiles but also very predictable in drop and wind drift.
Of course the accuracy wasn't that good, but theoretically a 45-70 can be just as accurate as .408 Chey Tac if there is little wind. All you have to do is make good consistent rounds and compensate for the drop and at those ranges the spin of the earth. There is little more to it.
But one must keep in mind that such accuracy is not needed on the modern battlefield, shooting anything beyond 800yds is a trick shot in combat, even for the best snipers. If your calculations are off by the tiniest amount you're going to miss, and we're speaking about moisture, crosswinds, crosswind angles, temperature, altitude etc.
Its much wiser to get closer and be absolutely sure the first shot hits. Also positively identifying targets is absolutely impossible at those ranges, you'd need nothing less than a telescope.
Shooting super long range is fun and very impressive but not practical, when things get serious keep it as easy and safe as possible. Don't get carried away with long range accuracy.
Oh, and about the video, how in the nine hells can the shooter see and aim at a 36 inch wide plate at 3600yds with just a 25x magnification?
