adjective. Of or relating to aerodynamic flow or flight conditions at speeds near the speed of sound.

adjective. just below, or just above the speed of sound

adjective. passing from subsonic to supersonic, or vice versa

adjective. (of speed) having or caused by speed approximately equal to that of sound in air at sea level

"Bullets become unstable when they travel through the "transonic" stage (where the bullet goes from being supersonic to subsonic) the reason for this as best as I understand is due to the change in the way the air flows over the grooves in the bullet that the rifling engraves in its surface."

sound barrier  

hypersonic  plasmasonic  subsonic  supersonic