Actually, if you take the same case, say the 30-06 case, and neck it to different calibers, say .30 cal., 8mm cal, .338 cal and .35 cal, and load them with the same bullet weight to the same pressure, say all at 60,000 psi, then the larger diameter bullet will always go faster at the muzzle. This is because the base of the bullet is bigger, has more surface area for the pressure to push against, and thus will accelerate faster, since more foot lbs of energy are pushing against it. Any good physics text book will explain this somewhere in it.
