I have never had the chance to hunt grouse so far, but what you're doing up in the wilderness sounds very cool!
A few questions to clarify:
(1) What impact energy (not muzzle energy) do you need on grouse? (For headshots the necessary impact energy is negligible, for body shots a different story.)
(2) What caliber would you consider minimum?
(3) What would be the outer limit where you'd want to attempt a shot?
The reason I'm asking is that it seems to me you're hoping to
get 3 things in one airgun that are mutually exclusive :
▪ Short (pistol!)
▪ Quiet
▪ Powerful (to be confirmed upon your answers to the questions above)
Most of my own airgun purchases suffer from similar high expectations like your grouse gun quest!
To help me and other understand this predicament I made up a little
infographic.
It's message: You need to lower
one of your 3 requirements somewhat.
The infographic includes some remedies — tips and tricks how you can still get pretty much what you want.
➔ Once the power/ caliber/ range questions are clear we can look for specific guns that could work.
I have some ideas.
Matthias