Why are you only considering CO2 models? The best airguns, as I recall, are the spring-piston (not to be confused with pump) variety popular in Europe.
I used to own a Feinwerkbau air-rifle that had a beautiful finished walnut stock. You loaded it like a coach gun - which simultaneously activated the spring piston. I got about 750fps out of it using .177 pellets. Had a scope and everything.
Beeman is a popular US distributor of many European air guns - rifle and pistol. Check out this website:
Beeman