The relative motion of a bullet heading straight toward your eye or straight away from your eye is, absent gravity, zero. The bullet would appear to get smaller or larger over time, depending on whether it is going away from you, or coming toward you, respectively. But, it would not otherwise appear to move in any direction. Add the effects of gravity or wind, or precession of the bullet and some motion would be perceived -- ie, the arc of the dropping bullet. But human visual perception is complex and the brain "fills in" a lot of information that, physiologically, the eye is not capable of detecting. This is particularly true for the visual detection of motion. What you "see" and what you think you see are sometimes quite different.