So what is?
Nothing. And everything.
For hunting purposes, slight to significant variations in centerfire cartridge shapes contributes nothing to inherent accuracy. Wall taper? Doesn’t matter. Total length? Doesn’t matter. Shoulder angle? Doesn’t matter.
BONUS VIDEO: HOW ACCURATE ARE SLUGS?
Changes in the shapes of centerfire cartridges contribute next to nothing toward inherent accuracy. The case is not magic, it’s just a convenient powder reservoir.
What matters is a consistent powder charge, plus bullet and barrel concentricity. Combine those with a true (straight and square) action bedded properly in a stock that doesn’t shift to alter pressure against the barrel, nor torque the action, and virtually any centerfire cartridge will be a consistently precise, accurate performer.
Nitpickers might argue against this, noting that the .308 Win. has won more target competitions than the 7mm-08 Rem. or .270 Win., etc. But that might be just because more target shooters chamber for the .308 Win. than any other. And even if the average .308 Win. rifle outshoots the average .270 Win. by .005 inch in a target match, what game animal is going to notice?
That’s the main point here: Don’t fuss and worry over which cartridge to buy for big-game hunting based on inherent cartridge accuracy. There isn’t enough to matter. Any rifle that consistently throws bullets minute of angle (1.047 inches at 100 yards, 2.094 inches at 200 yards, etc.) will park a bullet within 2.5 inches of where it is aimed at 500 yards. That’s well within the vital zone of even the smallest African antelope, which is considerably smaller than our standard-issue coyote.
If the above is true (and I have found it to be so after shooting and hunting with hundreds of rifles chambered for dozens of cartridges), why do so many hunters argue, puzzle and agonize over “the most accurate cartridge?” Human nature, I guess. We love to get the best. But in this case, the best has much more to do with everything about the rifle and ammo except for the cartridge shape and caliber.
Buy your centerfire rifles based on what you want the bullets to do. Shoot fast, flat and far? Land at high velocity? Drift minimally in the wind? Cost less per shot? Impress your friends? There are plenty of reasons to buy one cartridge/rifle over another. Inherent cartridge accuracy just isn’t one of them.