A muzzle flash is the visible light of a muzzle blast, which expels high temperature, high pressure gases from the muzzle of a firearm. The blast and flash are caused by the combustion products of the gunpowder and any remaining unburned powder mixing with the ambient air outside the barrel. The size and shape of the muzzle flash is dependent on the type of ammunition being used and the individual characteristics of firearm and any devices attached to the muzzle (such as a muzzle brake or flash suppressor).
Muzzle flash can be broken down into five distinct components:
1) Muzzle glow is a reddish glow that is visible before the bullet leaves the barrel. Muzzle glow is created by superheated gases that have leaked past the projectile and have exited the barrel.
2) The primary flash is caused by propellant gases exiting the firearm behind the bullet. Although amongst the brightest of the flashes, the heat of the primary flash dissipates quickly and thus is no longer visible.
3) The intermediate flash is caused by shock waves created by the high speeds of the escaping gases and projectile, and appears as a reddish disc shape in front of the muzzle.
4) The secondary flash appears farthest from the muzzle as a large white or yellow flame. Secondary flash is caused by the mixture of fuel-rich gases and oxygen in the atmosphere surrounding the muzzle.
Following the dissipation of the muzzle flash, partially unburnt powder or other heated materials can be ejected from the muzzle and appear as sparks.
Muzzle flash, particularly the long duration secondary flash, is an inherent problem in most firearms. Due to its brightness, muzzle flash can temporarily blind the shooter, or give away the shooter’s location, especially at night. Ingestion of the muzzle flash from aircraft mounted guns has also been implicated in compressor stall and flameout, causing loss of aircraft.
Flash suppressors attempt to suppress the flash mechanically, by interfering with the shock wave using either a cone or a series of slots at the muzzle of the firearm. However, since the primary cause of the secondary flash is combustion of hydrogen and carbon monoxide, chemical approaches are also used. In World War I, bags of sodium chloride (table salt) were placed in front of the propellant charges of artillery to suppress the flash. Addition of a few percent of alkali salts to the powder for flash suppression is common, typically salts of potassium such as potassium chloride, potassium sulfate, potassium carbonate, and potassium bicarbonate. In both cases, the salts act as catalysts, and interfere with the hydrogen-oxygen combustion to reduce the muzzle flash. The side effects of the alkali salts is a reduction in power, an increase in smoke, and fouling and corrosion of the firearm and nearby equipment (a significant concern with aircraft guns). Ammonium chloride and ammonium nitrate salts have also been tried with success.
Here’s a quick video clip of a blast from a short barrel (Diamondback DB-15) exhibiting a nice, bright muzzle flash: