Free: Contests & Raffles.
Head shoot them always. #6 shot size.
The 28 gauge seems like it would be the perfect grouse gun. I may have to get one someday.
6's and 4's would be my choice.
Grouse are not hard birds to bring down. a 1 oz load of 6's or 7.5's is plenty, as long as you do your part. If all I had was a full choke 20, I would give polywad a call and buy a box of spreader loads, to turn your full choke into a modified. If you do manage to connect at close range with a full choke, that is not a bird I'd want to eat. Open up that pattern: you'll kill a lot more birds.