From what I glean from the long range benchrest folks, the larger 30 calibers take home more bacon than the others at 1000 yards. For shoulder fired rifles in long range prone matches (600 yards plus), the 26 and 28 calibers are the favorites these days except for Palma matches where the .308 Win. or .223 Rem. are the only cartridges allowed. All the popular long range cartridges for both benchrest and prone matches will shoot with equal accuracy; under 6 to 7 inches at 1000 yards. Those with less recoil are easier to shoot accurately off the shoulder; doesn't matter with free recoiling benchrest guns. Once in a (great) while, one will cluster 5 shots inside 2 inches at 1000 yards and everybody's ga-ga over that marvelous accuracy. Few, if any, realize those extremly-tiny few-shot groups happen only when all the variables cancel each other out.
There's no accuracy difference between stiff and whippy barrels for long range accuracy. If there was, then those 30 to 32 inch long, slender, whippy 5-pound Palma rifle barrels would not shoot just as accurate as shorter, thicker ones used in long range benchrest rigs. The most repeatable thing a rifle has from shot to shot is its barrel whips and wiggles at exactly the same frequency for each shot. And at the longer ranges, whippy barrels may well shoot ammo more accurate that stiffer ones; especially when that ammo's got a wide velocity range. The Brits proved this over a century ago with their .303 Short Magazine Lee Enfields and .303 Lee Metfords.
Most folks I know of will pick a cartridge for shoulder fired rifles that's more accurate to shoot than one that bucks the wind a little better.
Another thing to consider is accurate barrel life at long range; no worse than 6 to 7 inches at 1000 yards. The popular 6.5x.284 and 7mm SAUM round popular in prone matches has a barrel life of about 800 rounds for best accuracy. The .308 Win.'s barrel life is about 2500 rounds. .300 Win Mags and others of that size go about 1200 rounds for best long range accuracy.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master