agree it must therefore be a defect or shape that disturbs the airflow....
Yes, you will not hear the bullet approaching - hence, if you HEAR it... the danger has passed by
The physics involved is very interesting;
if the target is 1000m away and the proj is traveling at an average of 3000fps = 914.4m/s
Therefore it takes the bullet 1000/914.4= 1.09 seconds to reach the target. For the sound of the hit it will then take another 1000m / 343m/s(speed of sound) = 2.92 seconds for the sound of the hit to 'return' to the shooter.
Therefore, from when you pull the trigger, it will take 2.92 + 1.09 =4.01 seconds for the shooter to 'hear' the hit.