Sub-sonic ammo: The bullet is traveling slower then the speed of sound, but the gasses escaping behing it are not----> boom.

Pellet rifle shooting 1400 fps: The pellet is traveling faster than the speed of sound----> crack.

Pellet rifle shooting 900 fps: pellet traveling slower than the speed of sound, gasses behind it are also----> dead squirrel, ppppsssssshhhh.

Physics.