Except that a .308 doesn't have a short case- its a bit shorter than a 30-06, but looks looong compared to any of the PPC cartridges which are about as wide as they are long in the body. A short fat case wouldn't explain why a .300 magnum or .338 lapua magnum is a better long range cartridge than the .308, let alone that the .223 is at at least as accurate as the .308. If you want to go shorter, one could claim by the same reasoning that the 7.62x39 cartridge should be much more accurate than the .308.

One can make a case for PPC cartridges being short and fat and accurate, but it just doesn't wash with the .308, its a completely different bird altogether.