Shot through a rifled barrel is super hard to make work. I've fired .22 shot shells through a rifled .22 as well as buck shot through a rifled slug barrel. The pattern is a donut that completely avoids what you were aiming at. You'd almost need to aim to the side and hope the donut ring puts at least one pellet where you want it.
As for the gamegetter, love mine, but have been thinking of experimenting with it: What if... Instead of resizing shot to fire, you took two fishing sinkers and crimped a 6 inch piece of 50lb or more line between them. Then pounded them into wider disks and ran them through the resizer, and seated both of them in the casing with the line carefully spooled and sandwiched between. When fired, (my hope is) the rifling and centripetal force would cause each sinker to fly out and act like a bolo. You could just aim for the neck of the grouse, and as long as you're within 6 inches, it should slice the head right off.
Should be easy enough to test on paper or a stalk of celery. Now where'd I put my tackle box...