High Speed Shadowgraph of a supersonic bullet and fragment in flight
A bullet flying at almost Mach 1.5 (1.5 times the speed of sound) causes air to be compressed as it pushes its way through "thin air" causing a classic "V" shaped shock wave to appear reminiscent of the "bow" wave of a boat moving on a smooth water surface. This wave is made visible by the shadow it causes to appear on a piece of film at the time that a 1 microsecond duration flash is fired in response to the shoock wave passing over a sensitive microphone. The bullet is clearly a supersonic event. The small particle to the left, a piece of gunpowder or a fragment of lead from the bullet, does not exhibit such a pronounced shockwave and it is traveling in what is called the "trans-sonic" range, namely very close to the speed of sound. When the "V" shaped wave travels over our ears we hear the "sonic boom" associated with a plane traveling faster than the speed of sound.