RE: Why are some calibers more accurate?
While I agree with many of the remarks so far, one must think about consistency. A change in temperature, from a cool morning when a rifle was sighted in to a hot afternoon when a shot was taken at a target, might affect the performance of some cartridges more than others, so the impact point might change more. I don't know if fast and slow powders deliver the same performance in changing circumstances. This would not affect accuracy on any one shot. Here are other considerations: given a powder scale of a certain accuracy, the errors in weighing that are introduced will have a greater effect on a small cartridge case than a large one, will they not? Doesn't the same hold true for tolerances in bullets and cases - i.e. the smaller the caliber the tighter the tolerances must be to get equally consistent shot to shot performance?