Stupid Noobie question about rails.
So I finally got around to buying an AR-10 (I know I should have built one, but you know the saying about hindsight being 20-20).
I have a question about the rails on these modular guns. I see it has a sort of dove-tail shape like many regular long guns have, and understand how the site rings clip on to that shape.
What I don't understand is what the square-ish blocks are for above that?