On the 'full sized" rifles (Classic Green, Blue Star,etc) the flash hider is 21.5mm (the NATO standard is 22mm) with rifle grenade ring, it is a machined prt of the barrel and non removable.The Carbines have a removable F/H of either a bird cage design or Vortex type with a different thread pitch than the AR15 FOW. The XCR uses standard AR15 specs for it's flash hider