Flash Hiders are available for all non-bolt action rifles in the game, excluding the Gewehr 43, which has a suppressor instead. It is unlocked for a weapon after completing that weapon's Marksman I challenge.
The flash hider greatly reduces muzzle flash, and makes the shooter invisible on radar, meaning it is effective for stealth classes. It does, however reduce damage over range, which makes rifles weaker at medium range. It also greatly reduces the firing sound, but not as much as a Suppressor.