This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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

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

adj. passing from subsonic to supersonic, or vice versa

adj. (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   

subsonic    plasmasonic    hypersonic    supersonic