Depends on the gun, At 100 yards some bullets will impact in a fraction of a second. A 12 gauge slug will take longer to get there. I have shot deer that are walking but I always try to stop them first. A super fast bullet will get there and the margin of error from the deer walking is probably less then what you most people can accurately shoot. Just saying some guns might have 3 inch or more group at 100 yards and the target would move less then that, again depending on the speed of your bullet.