I don’t have a dedicated grouse gun. I found that my 12g Browning A5 mag is too tight for them and doesn’t leave much grouse for eating. It’s a full choke, fuller than most.
I have an A5 light that works a bit better, full choke, still tighter than I’d like. I have a short barreled open cylinder 12g that works great close up, but its range is limited. I’m sure that need to buy a new gun, I’m thinking modified. Maybe 20g.