Above is a picture of a bullet as it travels at supersonic speed. As the bullet travels through the air it traps air at the front of the bullet due to its supersonic velocity. This air at the front of the bullet forms a shock wave which you can see visualized here. It is this shock wave that is part of the noise you hear when a gun is fired. At the rear of the bullet you can see a low pressure area and the eddies that formed there.
Andrew Davidhaz – Rochester Institute of Technology