The Vortex flash hider uses a patented 15 degree, helical flute design to eliminate muzzle flash. Used by US armed forces worldwide as well as US Dept of Homeland Security and law enforcement across the nation. Machined from ordnance grade 8620 steel and finished with a mil-spec phospate. Designed not to loosen under sustained fire. This Vortex uses the military's standardized 30 caliber thread pattern of 5/8"-24 x 0.735" (depth), and is compatible with most modern bolt action and semi-auto military-type firearms up to 30 caliber. Also ideal for use with the 6.8x43mm SPC cartridge. 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.