Weatherby magnums are generally at the highest velocity for each caliber bullet their cartridges for factory stuff. But, they aren’t designed for the high ballistic coefficient bulllets, except the new 6.5/300 or whatever it’s called.
Velocity alone is important out to about 500, after that the ballistic coefficient starts to matter more as the drag on the bullet slows it down.
A flat base 60 grain .243 bullet will get to 500 yards faster than a 105 grain .243 bullet, but the 105 grain bullet is more aerodynamic so ultimately there is a point at which the 60 grain slows down but the 105 keeps going because of the difference in drag.
For long range, heavier high ballistic coefficient bullets for caliber are better.
Similarly, if you shoot a 140 .308 bullet at the same velocity as a 140 .264 bullet, the .264 bullet has a higher ballistic coefficient because it is longer. The .308 will have higher velocity but it also slows down faster.
If you want to shoot long range, the best cartridges are ones that are designed to shoot a high BC bullet. High BC bullets require a faster twist barrel, typically. So, a .260 REM, though very similar to 6.5 Creedmoor, typically doesn’t have the barrel twist or factory loaded ammo for high BC bullets.
Long Range is what has spawned many newer cartridges that are factory designed for the bullets and improvements in case and powder design. Those are the newer cartridges like 6 creedmoor, 6.5 creedmoor, 6.5 PRC, 300 PRC, the Noslers, 6.8 Western, 280 AI, etc.