Hi ;) i'm looking for a way to calculate the coordinate of the impact of the bullet on a target! ideas?
yes, you need multiple impact sensors (at least 3) and reverse calculate the timing of the triggers to the impact. Non trivial hyperbola math.
please read this thread from 2011 - http://forum.arduino.cc/index.php?topic=52583.0 - it discusses the math and an alternative solution. If there are questions thereafter, post them as explicit as you can.