The muzzle velocity of a bullet can vary from about 1080 f/s to 5910 f/s. It takes a human being about 0.1 s to recognize something moving and another 0.1 s for the signal to get form the brain to the muscles.
Thus, even a slow bullet would travel almost 2000 feet (allowing for friction) before a human could even twitch. Obviously the human eye could not see a bullet at 2000 feet, let along discern it's motion.