All I use is Black Cloud, 3.5" BB for geese and 3" #4's for duckies. If I am hunting both at the same time, its #2's.
At 30yds, most shots are through and through, and rarely is a second shot needed. I shoot for the neck where it meets the body, there's more of a chance that I am going to hit something vital. If you just shoot at the neck, your pattern may not be tight enough to put a lethal amount of pellets in there. Change your point of aim, or change your choke. I am shooting a mod choke currently.