I use 12ga IC, early in the season. Modified choke later on. Always shoot #5 copper plated.
If using a 20 gauge, will go with 3" shells, #5 copper plated.
In Iowa, wild pheasants later in the season are a tough bird to bring down at distance. No sense in being under gunned, and losing a bird.
By the way, I ALWAYS, hunt with a labdrador . You are wasting your time hunting pheasants in Iowa without a dog.