I use either 20g or .410
If you're not walking miles to hunt your grouse, then fine, 12g will work, but my opinion is that it's a bit too much gun for upland birds if your covering territory on foot. It makes a big difference after a short while!
I don't really like shooting them on the ground, so .22 is out for me. Nothing wrong with it, just not my preference.
I have full over mod and that's all.