It's a ring, lined up with 3 lines/bars/whatever-you-call-it. There's a mod I believe is called RL_Iron_Sights.
If you use the 'aim' option (Rclick) you will zoom in on the sights of your weapon. it's a code thing, those weapons were already in place in the core game. Features a custom iron-sight model, custom yellow laser effects and custom textures. There is one main thing about FNV guns that Fo3 guns don't have, and that is the #Sighting Node (node) for making it so that weapons will sight properly in the game.
