The very reason they have a pattern board and a grease roller at most Sporting Clays ranges worth a flip. If you do not have a gun that is fit to you or know where in the world a gun shoots you are just pizzin in the wind.

Back to the original question, open chokes for grouse, especially in the thick eastern woods. A modified is mostly too tight. However, you can always have a fixed choke opened up.

“To expect defeat is nine-tenths of defeat itself. It becomes a self-fulfilling prophecy. It is best to plan for all eventualities then believe in success, and only cross the failure bridge if you come to it."
Francis Marion - The Swamp Fox