From The Next Big Future:There is a patent for a electronically enabled bullet dodging capability. If a sniper is firing from 2500 meters then there is 4 seconds to move out of the way if the bullet can be detected as it emerges from the muzzle. (H/T to Firearm blog
The knee reflex takes about 20 milliseconds to fire. Bull riders can have head accelerations of 26-46 Gs (258-450 m/s**2). So 40 m/s**2 head acceleration should be safe, although it could be necessary to move someone faster and get whiplash instead of a bullet hole. Moving about 15 centimeters would often be enough to get out of harms way.
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