This article or section should be merged with Flash suppressor

A flash hider, also known as flash eliminator, is an attachment on the end of the barrel of a weapon that is intended to reduce the amount of "flash" given off when firing. They became common on post-World War II assault rifle designs, and are almost universal on these weapons today.

Earlier rifle designs from the 19th century tended to have long barrels that gave the bullets accuracy at what would today be considered very long ranges, perhaps 1,000 yards. A side effect of the long barrel is that the gunpowder is completely burnt before the bullet leaves the barrel. However if the same weapon is "cut down", as is common in cavalry and jungle-combat versions, the bullet would often leave the barrel before the powder was completely burnt. In this case the unburnt powder would ignite in the air, giving off a bright flash.

When barrel lengths dramatically decreased with the introduction of various assault rifles, this flash became a serious problem. Limiting the amount of powder to what the barrel could burn is generally the idea, but differences between individual cartridges mean that some will always have too much. 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 hiders reduce, or in some cases eliminate, the flash by rapidly expanding the gasses as they leave the end of the barrel. Although the overall amount of burning gunpowder is unchanged, the density is greatly reduced, as is the brightness of the flash.

A number of different flash hider designs have been used. The simplest is a cone on the end of the barrel, which was used on the jungle-combat versions of the Lee-Enfield. More modern solutions tend to use a "basket" with several slits or holes cut in it, as seen on the M-16 and other small-bore weapons. Flash hiders are often used as a muzzle brake as well, by cutting slits only in the top of the hider, thereby providing some down-force on firing as the gasses escape upwards.