People at my school actually think bullets travel at ultra-uber-supersonic speeds. They think that once you pull the trigger, the bullet gets to your target instantly.
Like shooting a plane in MW2, pull the trigger, the bullet is there.
You have to lead moving targets, or you won't hit them. Except that depends entirely on how far the target is. Plus, bullets lose energy after a certain amount of feet, so you can't shoot an M9 and hit a target one mile away by aiming directly at it.