it depends on the model, the ruger 5.56 with the front sight perminatly attached on the barrel just in front of the existing hand guard you will need to replace that sight as it is also the gas block you will need to replace that to or buy a two peice handguard that fits over it. at the upper receiver yes this will work as ruger made it standard millspec but for that rifle ive only seen one or two that dont have that sight on them and you WILL NOT be able to use this hand guard on it without significant modification. find a two piece handguard its fast ez and will take you all of 20 minutes.