Shock waves from explosions are incredibly weird things. They travel at supersonic speeds, are really nonlinear, and though they measure only nanometers in amplitude, they produce intense jumps in atmospheric pressure: A wave travelling at Mach 2 can produce 4.5 atmospheres of pressure and destroy the human hearing system. Indeed, shock waves from the Hiroshama and Nagasaki nuclear bombs, as well as the traditional-means Oklahama City bombing, that caused much of the serious physical damage in those attacks.

It's traditionally been hard for scientists to visualize shock waves -- until now, when a team of Penn State scientists developed a technique that uses high-speed videography to capture the nearly-instantaneous atmospheric pressure changes of explosions and make them visible. An article at American Scientist explains the technique, and includes some mind-boggling pix of the shock waves created by gunfire.

The article concludes with a very cool summary of the psychoacoustic dimensions of being shot at:

The first visible phenomenon at the muzzle is the emergence of the bullet-driven shock wave, followed immediately by the bullet itself. Then the propellant gases, the products of gunpowder combustion, exit and expand tremendously as they transfer from high pressure inside the barrel to one atmosphere outside. This rapid expansion behaves like an explosion in pushing the air out of the way and thus generating a strong spherical shock wave, or muzzle blast. The "bang" of a gunshot is almost always caused by this muzzle blast.

If you are unlucky enough to be shot at but lucky enough to be missed, sometimes you hear instead the sound of the bullet itself. Inertia keeps supersonic bullets moving at high speed, while the muzzle blast rapidly decays in strength like the spherical shock wave from an explosion. So the bullet inexorably pulls ahead of the decaying muzzle blast, trailing oblique shock waves. These shock waves produce the sensation of a sharp "crack" as the bullet passes, followed later by the "bang" of the muzzle blast. This sequence varies with timing and the hearer's position with respect to the bullet's path, making it very difficult to determine the direction of gunfire from its perceived sounds.

(Arg -- I've lost the email of who sent me this item. Please email me so I can attribute this to you!)