A 12g with removable chokes, doesn’t matter if it’s a single shot, pump or semi. The grouse won’t know the difference, 12g shells are everywhere and still relatively cheap. Removable chokes will allow you to shoot steel or lead and pick your appropriate pattern for your distances etc.
I agree with this choice, example, I have a 12ga Browning Maxis with synthetic stock in 3” chamber & 28” bbl. It is light to carry, kick is very mild with target ammo in which 1 1/8 of 71/2 @1200 fps will down any Ruffie. I use skeet to ic choke for the Ruffies and shooting skeet. I can also use this gun for waterfowl using 3” steel, no need to punish yourself with 3 1/2 shells. Truly a do anything shotgun.