Moss 500
I've spent long years hunting Ruffed Grouse in the deep woods of northern Minnesota. Back a million years ago we used to have these long barreled shotguns that were more appropriate for duck hunting but back then that's what a "hunting shotgun" was supposed to look like.
In the past 15 or 20 years we've learned a thing or two. (1) Short barrels are better than long barrels. I personally prefer an 20 inch barrel with screw-in chokes. (2) Light is better than heavy. (3) Something that you don't mind getting dented, scratched, rained-on and muddy. After all, it goes on an ATV ride. (4) You will never need a full choke for Ruffed Grouse.
Mossberg 500 with 20 inch barrel, screw-in choke and black poly stocks. This short, black shotgun scares all the Jim Zumbo's (I take no glee in getting funny looks from fellow hunters, btw) but it does the job better than anything I've found for the realities of how my family hunts. This is a highly specialized configuration that does not translate well to Turkey Hunting, duck hunting, or quail hunting. It is a grouse set-up.
A nod of the hat to all those "coach guns" - nuthin' wrong with break-action just as long as it's not a collector's piece best left in the safe. A friend has a side-by-side, short barreled deal that he got for less than $ 200.00 - perfect for grouse. 16 or 20 or 12 doesn't matter ... barrel length and ruggedness are more important.
I love Ruger Red Label's over/under - great grouse guns - but they cost so much and are so beautiful that they are only for taking serene October walks. Same deal with Weatherby Athena and top end luxury guns ... doesn't belong anywhere near the woods but is better suited for shooting clays in a sport coat down at the club.