The Rice University Neologisms Database

The sound of a bullet bouncing off armor plating. The sound ‘kapwing’ is similar to the sound a bullet makes as it reflects off a medium such as armor plating. Because the bullet is traveling near the speed of sound, its Doppler shifted sound is heard, resulting in a higher frequency (ka) as it approaches the observer (who is standing at the collision point). The collision is represented by the ‘p’ (or ‘pw’) sound, and as the bullet moves away from the observer, again its Doppler shifted sound is heard, this time at a lower frequency because the bullet is moving away from the observer (‘wing’).