My newly purchased ar10 has roughly a 18 in. free float rail on it. The free float is octagonal with several small holes that are threaded along the sides. It only has a picitinny rail on top but i was wondering if these small holes allowed a picitinny rails to screw in or if they were for something completely different.
http://s44.photobucket.com/user/Noah_Bridges/media/1480629968108-99379225_zpso7p1b2na.jpg.html?sort=3&o=0
http://s44.photobucket.com/user/Noah_Bridges/media/1480629921360-2100871636_zpshbyw4fcn.jpg.html?sort=3&o=1

That's exactly what they are for.  You put the length of rail section you want where you want.  Totally customizable.   You do need to stick with the rail sections offered for sale by the maker of the forend because so far as I know, there is no standard for rail section bolt spacing.   YHM, TROY, MI...all offer forends of this type along with their rail sections to go with them, so determine whose forend you have and order their rail sections.

I bought the rifle second hand and have no idea who manufactured it