I read that the speed of sound in air at 20 degrees celsius is about 1126 feet per second. When I shoot my rifle, there is a noticeable crack which I believe to be the 'sonic boom'. I also have 'sub-sonic' ammo that does not cause the CRACK, because the bullet is traveling slower than the speed of sound. My question is, if I shoot an Air Rifle (bb gun) that shoots bb's at an advertised rate of 1400 feet per second, why is there no CRACK (sonic boom) ? Is there a MASS requirement for me to hear the boom?