I'd have to disagree with you there. The sound that you hear is the explosion of the propellant contained within the chamber of the gun only. The bullet traveling at say 1100 fps is well above the sound barrier, but without propulsion, there is no noise associated with it, and with the small size and aerodynamics does not displace enough air to create the "boom" you would associate with it. I'm just using my memory here, but I believe that the speed of sound is 760 fps
Disagreed for discussion only, maybe I am off base here