For CO2 power, the Crosman model 2240, hands down.
Readily available, and there is a LOT of info available online for tuning them up, and making them even more accurate. Most are shooting well under the 500fps limit, stock, and some minor tuning can bring them up to just under the limit. .22 pellets are heavier and carry more energy downrange, and fly plenty flat enough, for grouse range.
The Crosman punp-up guns are not affected by temperature, and the only thing you need to carry is pellets. The current carbine "Backpacker" model is derived from the Model 130/1300/1377/1322 series of multi pump pneumatic pistols. Good stuff. I like the older ones. More metal, less plastic.
Big bonus of the pumpers, is that you can pick and choose for your power levels by varying the number of pumps.
Again,I'd choose a .22 cal. More smack downrange within the 500 fps limits.
Again, you can find a lot of mod info online, including stuff on bigbores, using roundball or cast/ swaged bullets, pushed to pretty reasonable velocities. $$$
Dunno what's in this years regs, but IIRC there was a very short blurb about no handguns at all that I saw in last years. Look under the list of things that are not allowed. IIRC, handguns were there.
Dig around online, and find the Canadian Airgun Forum. There is a subforum there on airgun hunting, with rundowns on the various regs in the provinces.
I figure if I cannot carry an air pistol, tucked away, and have to carry a rifle, it might just as well be a rimfire.
Take a look at the Hammond GameGetter too. Uses powernailer loads to push a sized roundball out at subsonic velocities (depends on the loads used) and they are pretty quiet. I have whacked a fair number of grouse with one.
Cheers
Trev