[ujiro]

The ring and barrel nut are indeed different parts, Acid_Snake is correct. Take of the bottom two pins and the middle pin on the side of the gas block (larger pins than the little one on the top, that little one just keeps the gas tube in place). Not sure what that other pin is. How does it move?

Once you take the gas block off then undo the collar (the part you pictured before that you can see), and then the rail will come off. Then you can take the barrel nut off with either the proper tool (an armorer's wrench), or some channel locks, or some pliers. However you want to/ are able to.