I used to use 3 inch steel #4's going 1550 fps...
I now use 3.5 inch steel #2's going 1550 fps...
Why... Because the #2's in a 3.5 inch shell gives me almost the same pattern density as the #4's in the 3 inch shell... So, if geese are involved, the #2's hit harder. Oh ya, I use an Improved Cylinder choke which gives you roughly a Modified choke when shooting steel...
Cheers
Jay
P.S. BB will kill 'em dead, IF you can hit them with a couple pellets... IMHO, guys forget about pattern density & shoot very large steel pellets that allows ducks to 'fly through' a shot pattern with few hits... Flying away to die elsewhere...