The Smaller the Cartridge, the Smaller the Groups
If you go to a typical 600-yard (medium-range) match, the typical cartridge will be something like a .260 Remington, 6.5/284, or 6.5 Creedmoor--140-grain bullets at 2,700 fps to 2,900 fps. Pretty puny, huh? But serious marksmen have known for years that burning lots of powder and getting gobs of feet per second makes it harder to get 'em all in the X-ring, not easier. If you sell your .300 Thunderf***er and get a .308, you'll immediately shoot much better.