I will agree that most hunters are not candidates to confidently make a shot past 250+ yds but yes, some exist.

At distances past 150 yds shooting aids help immensely such as a scope, shooting sticks, an accurate rifle, and a scope with distance correction device in the reticle. A laser range finder is needed if the bullet trajectory changes substantially at a particular shooting distance and if shooting up or down that angle + distance must be considered as the trajectory is altered. Knowing the horizontal distance to the target(an opti-logic ranger) is of great benefit and addresses that issue over distance. In other words, with distance game shooting becomes more of a technical affair in addition to having no wind, no game movement etc.

But for those who have these aids the long shot is possible to 450 yds. Past that I feel the horizontal drop of the bullet makes long distance shooting an ethical problem. Hits can probably be made but whether they will be humane killing shots is questionable. The drop between 400-500 yds for a big magnum with a 100yd zero can be 18-20 inches vs a drop of 30 inches from 500 to 600 yds.

Which cartridge is the best?

The 338-378 WBY/338 Lapua are the ultimate long distance elk /moose rounds IMHO, --