Decoy them in close. Then you could use 2-3/4" 20 ga. with #4 steel and IC choke.
OH YEAH!!
over decoys, my 20 gauge O/U is death top ducks and geese!
Ii use 2 3/4"#4's sometimes, but also 3" #2's
My son uses 12 gauge 2 3/4" steel 2's or 4's at 1550 FPS in a 870 pump with no problem at all, and my partner uses the same out of his 1100 Remington.
I'll stick with my 20 gauge however!
Cat