Transsonic is 'bout .75 Mach to 1.20 Mach. Below the speed of sound, some air may be pushed to Mach 1.0. Slightly above the speed of sound, some air will not make it supersonic and will keep the bullet from experiencing full supersonic effect. So in the transsonic range you will have supersonic and subsonic air. There is a boundary layer between the two with a shock wave. The boundary layer might not be able to withhold the shock wave. Bad things follow....

At least that's what I THINK.