I'm about to start a ar build. I have the lower the upper the barrel the gas block and tube the forward assist the dust cover the butt stock the flash hinder the pistol grip. Now I'm looking for a key mod rail. I've watched plenty of videos on how to instAll it. But I have one question and I know this is probably a dumb question. But what are the thread on the inside of the rail for it it doesn't screw into anything