The Vortex Flash Hider uses a patented 15 degree, left hand helical flute design to eliminate muzzle flash. Used by US armed forces by US armed forces worldwide as well as US Dept of Homeland Security and law enforcement across the nation. Machined from high quality steel with a nitride finish. Designed not to loosen under sustained fire. Notes:Due to the design of the Vortex Flash Hider, no crush washer is needed to install this sight and it is not recommended that one is used.