There is no one answer. Try different loads and see which ones you are competent with then do penetration tests.
The best shotgun shell for shooting ducks is a 3 to 3 1/2 inch 12 gauge with number 2 shot.
Shotgun
A Mossberg 410, home defense, or a 18 inch barrel 20 gauge Mossberg 500 youth model with accu choke . Between $250.00, and $350.00
It will depend on what you are hunting and which brand of shell your shotgun patterns best.
shoot it with a sawn off shotgun
2012 remington express max
Best left to a gunsmith
A custom fit Purdey double shotgun
Best left to a gunsmith.
I have the Gaugemates for 20 and 12 shotguns 12-16, 12-20 and 20-410. They work good and won't crack, they are made of Stainless Steel. Cost less than 60/pr. The only problem is time, time to get out the shell out from the reducer. Best reducers were the Savage, I have 12-410, they don't do them anymore. Gaugemate have the gold ones, those will take the shell out with the shotgun's extractors. They are so expensive that is better to get extra barrel or even a new shotgun. I don't think the gold are worth it.
Best left to a gunsmith.
Best to ask a gunsmith for help.