When I hit the grouse woods I'm looking for an easily carried and maneuverable gun and I don't pay too much attention to gauge and barrel length. I hunt over setters and I only shoot at birds on the wing. I typically adjust the load to the quarry more than worry about what gauge.
I might take my Francotte 28 gauge at 4 pounds, 10 oz
It could be the early fifties solid rib Browning Superposed 20 gauge with 26" barrels
I might grab my J & W Tolley (I sure would pick this one if hunting with Pinfire), a 5 pound, single 29” barrelled 12 gauge from England.
If it's a nice day and the woods won't be too thick I sometimes grab the Purdey, a 12 gauge 25 1/2" 6 pound 3 oz open choked grouse killing machine.
The only consistency in my grouse hunting setup is me, my dog and a shotgun.