Back east, shooting black and grey squirrels with the odd fox squirrel thrown in, I use 1 oz. of # 4's and 5's from my little single 20 bore. When switching to the O/U 12 bore, I kept the same shot size, but upped the load to 1 1/4oz. field loads- 3 drams. Was usually good to just over or around 40 yards with the 12.
I preferred 6's for grouse, which were always flying dead away, flushing wildly 30 to 40yards out. At the longer ranges, the 7 1/2's were a bit light for heavy fall feathers- and also, too many pellets in the meat at closer ranges.
Here in BC, we oft times head shoot grouse with our moose rifles or with a .22 kept in the truck just for them as they peck for gravel on the logging roads on mid day drive a-bouts.
Taylor was with me, back in about 78 when I shot a grouse behind old Les Hawke's diggins on Walcott Road between Smithers/Houston. I had my moose charge in the .58, 140gr. 2F and a .575" ball. I guess I pulled a mite low - we saw some birds fly behind the smoke and Taylor said "you missed". It certainly appeared that way.
As the smoke cleared, we could see pieces of grouse and lots of feathers in the willow bush that was behind the grouse. Appeared my shot went dead centre and spread the grouse thinnly in the surrounding bushes and ground. One leg hung by a sinew strand wrapped around a tiny twigg, swinging slowly back and forth - while somewhat amusing, it left nothing for "fare" & an empty dirty gun.