I do not live in an area which has grouse. My experience is thus with quail, dove & woodcock. I have used both #8 & #7 on these. I never had a problem killing any of these with either size "IF" it was hit. If I m shooting 7/8 oz I will normally go with #8 for the extra shot. More than 1 oz I would normally go with #7 to avoid putting so many shot in the bird. For 1 oz I might go either way depending upon the gun's choke. Althoug I never kept records I know that one gun with which I had the highest percentage of birds killed to shots fired on quail & woodcock was a 12 gauge firing 1 oz of #8's at around 1125 fps. This gun had 28" Damascus barrels choked choke in both barrels & weighed a couple of ounces under 7 lbs. For some reason I never carried it on a dove shoot, though it should have been ideal for all except very high fliers.
This was some years back & shells were loaded with the old original DuPont HyScore (pre 700X) to around 8-8K PSI, never felt a need for any lower pressure. I foolishly sold it but it has since come back home, but by that time I was not able to get out & hunt as I had previously.
It was also a most excellent cottontail gun with same load but #6 shot. At that point my Dad loved rabbit hunting with a pack of Beagles, so I went with him a lot. He passed away back in 1996 but had been unable to hunt for a few years prior to that.
