I hunt ruffed grouse (and usually woodcock at the same time) in northern WI. The cover is typically extremely thick and shots are almost always under 20 yards. Most of the time, you have a very short window through which to shoot and a bunch of branches in the way.
The gun I use most often is an old beat-up 16ga Winchester Model 12. I purchased it with a full-choked barrel, which I later trimmed down to about 24". This made it easier to handle in heavy cover and removed the choke. I now get wide, even patterns that work great at the distances I shoot most grouse. For ammo, I use 7/8oz of #7 shot; although if I'm using a different gun I'll use one ounce of #7.5.
The biggest consideration for me when grouse hunting is having a gun that I can carry all day and always be ready to shoot. Grouse will pop up at the least-expected times and give you almost no time to react.
FWIW, at various times I've hunted grouse with: and NEF 12ga single, 16ga Browning A5, 12ga 870 w/ 21" rifle-sighted barrel, 12ga 870 w/ 28" barrel, 30" barrel Browning 425, a Beretta 391, a Benelli 20ga M1, and probably a couple others as well. The 16ga Winchester mentioned above has accounted for more grouse than the rest combined.
Frankly, if I shot it a little better, that NEF single is darned near ideal for it. Lightweight, easy to handle and I can only think of a couple times when I've connected on anything other than the first shot.