I bought Sig 556 Commando and it does not have rear iron site. I did not get any manuals with it. What site do I need to buy? I looked around, there are all kinds of BUIS sites for sale but which one of them is for Sig 556 and which one is good and reasonably priced? Also. How do gas tube and hand guards come off? I figured out the rest of the basic break down but not that part.