9/16-24 RH thread. This is an early FAL muzzle device for retrofitting barrels with the belgian long flash hider. It is a combination flash hider and muzzle brake; incorporates a bayonet mount, and is threaded for blank-firing adapter. The 22mm outer diameter is for rifle grenades. Rifle grenade retaining spring is sold separately. There is a hole for a tangent roll pin to prevent unscrewing. Parkerized