The front site base must be removed. You have to do this to be able to remove the old barrel nut, and replace it with the standard free-float handguard nut that comes with the FIRSH handguard.
To facilitate the removal of the Front site base, you must, of course, remove everything forward of the front site base. This means that all flash hiders and muzzle brakes must be removed as well.
If you have the type of weapon that has a permanantly attached flash hideror muzzle brake, this is not possible. This is why Oly does not offer a post-ban permanent brake so that all of their customers have the option to upgrade.
Thes are EASY to install, and you do not need to remove the barrel from the upper. Although, as stated above, you do have to remove the barrel nut and all things forward of it to install the new barrel nut. Therefore, once this is done, all you'd have to do is give the barrel a good yank, and it would come right off.
Easy to install, BEST rail handguard on the market hands down!