Rationale
Pre-20th century rifle designs tended to have longer barrels than modern rifles. A beneficial side effect of the long barrel is that the propellant is completely burnt before the bullet leaves the barrel, usually resulting only in a puff of smoke from the muzzle. However, if the same weapon's barrel is "cut down" (shortened), as is common in cavalry and jungle-combat adapted versions, the bullet would often leave the barrel before the powder was completely consumed, resulting in a bright flash from the muzzle.
When barrel lengths were dramatically decreased with the introduction of various short-barrel rifles and carbines, this flash became a serious problem during night-time combat, as the blinding flash would make it impossible for the shooter's eye to adjust to the darkness. Originally limited to "special purpose" roles, it was now expected that all infantry weapons would have short barrels with this problem, and thereby be of limited use in night combat. Flash suppressors became common on late-World War II and later assault rifle designs, and are almost universal on these weapons today.
Military flash suppressors are designed to reduce the muzzle flash from the weapon to preserve the shooter's night vision, usually by directing the incandescent gases to the sides, away from the line of sight of the shooter, and to reduce the flash visible to the enemy. Military forces engaging in night combat are still visible when firing, especially with night vision gear, and must move quickly after firing to avoid receiving return fire.
Limiting the amount of powder to what the length of a barrel can burn is one possible solution, but differences between individual cartridges mean that some cartridges will always have too much powder to be completely consumed, and the reduced powder load produces a lower projectile velocity. Muzzle flash can also be controlled by using cartridges with a faster-burning propellant, so that the propellant gases will already have begun to cool by the time they exit the barrel, reducing flash intensities. Faster-burning powders, however, produce less projectile velocity, which reduces the accuracy due to introducing more of a parabolic bullet flight path in place of a "flat" trajectory while also reducing lethality of the weapon by reducing the energy delivered on target.
Flash suppressors reduce, or in some cases eliminate, the flash by rapidly cooling the gases as they leave the end of the barrel. Although the overall amount of burning propellant is unchanged, the density and temperature are greatly reduced, as is the brightness of the flash.
A number of different flash suppressing designs have been used. The simplest is a cone placed on the end of the barrel, which was used on the late-World War II jungle-combat versions of the Lee-Enfield, the No. 5 variant, intended for use in the Pacific. More modern solutions tend to use a "basket" with several slits or holes cut in it, as seen on the M16 and other small-bore weapons. Cone-shaped flash eliminators are also evident on the Bren LMG and on the turret-mounted aircraft machine guns of British WWII heavy bombers, which were used mostly at night.
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