RE: What is a good duck gun?
It depends on what type of duck hunting you are going to be doing. For all intents and purposes a 2 3/4 in. chamber should be plenty. It's all i ever used in 50 years of duck hunting. And it is a great cartridge for clays and small game - using lighter shot size.
The next question would be what actions are you familiar with and / or do you like. Also get a gun that fits you. Any knowledgeable sales staff should be able to help you with that one.
Good luck and good/safe hunting.