Muzzle flash is a trademark of small arms fire. To understand why a weapon produces a fiery discharge, it helps to break down what happens when someone pulls the trigger of a gun. First, the trigger lever pushes the hammer backward, causing a spring to compress. As this spring uncoils, it drives the hammer forward. The firing pin on the hammer then strikes the primer, a small initiating charge incorporated into the ammunition cartridge. Upon striking, the primer generates heat to ignite the propellant, such as black powder. The propellant undergoes a process known as deflagration, which is something between burning and exploding. Deflagration produces a large amount of gas within the firing chamber, and it's this gas, under great pressure, that drives the bullet down the barrel. Once the bullet exits the barrel, the gas finally escapes, creating a flash of heat and light.
From a shooter's point of view, muzzle flash causes problems, especially at night. For example, an intense blast can give away a gunman's position. But a bigger problem occurs when the discharge blinds the shooter and diminishes his view of the battlefield. For soldiers, this can be a significant concern, which is why military rifles often come equipped with flash suppressors -- devices that attach to the muzzle and reduce the weapon's blast by directing the incandescent gases to the side.
Because flash suppressors are so common on assault rifles, gun-control advocates see this product -- or a threaded barrel designed to accept it -- as a military feature that would give civilian shooters an unwanted or unnecessary advantage. For this reason, assault-weapon legislation often includes language banning the use of flash suppressors.