As a new Darne owner, I would add a couple of things that puzzled me in disassembling the gun.
First, put the gun on safe and pull the bolt to the rear. If you reach underneath the bolt you'll feel a bump. Push that; it will make the bolt possible to lift up and off.
Second, the flat spring that holds the barrels must be depressed at the front of the spring, but at least in my gun, the movement is so slight it's difficult to tell you're depressing it. Added to that is nothing happens immediately when you press it. Then you can hit the barrels against the carpet and they'll come right off.
I really like my 20 gauge. As I've posted before, I saw a Darne in Viet Nam, a 16 gauge. It was a capture.