RE: 20 GA for ducks?
A 20 ga. shreds. I use to load 1 1/4 ounces of nickel #5's in a three inch case for my 870 and with the 20 guage shot string it was like shooting a rifle. I've never shot steel through it, but it's all relative.
Don't let anyone tell you that a 20 guage can't kill. There's plenty of ducks on that big pond in the sky that could tell you otherwise.
Boldt