I've got a number of CO2 air pistols, with the 2240 14" barrel being my most capable (between 500-600 fps). It's loud but superbly accurate--custom shop made that I bought used. However, I won't expect it to do well in weather below 70*, unless you kept the gun inside until shooting. Now, I have a Hammerli 850 gen 1 CO2 rifle (.177) that shoots 625 -675 fps that regularly kills squirrels when I do my part and hit the head. My longest shot was just over 25 yards, nearly between the eyes. I would suggest that you look at the .22 or .25 calibers for bigger game. After learning with the .177, I wouldn't hesitate to buy a quality CO2 or a multi-pumper in a bigger caliber. When I joined the forum, I thought it was all about the higher FPS. Now, it's about the right tool for the job (grabbing the right gun), the best pellet, and hitting the best POA for the kill.
Let us know what you get. Enjoy!!!
Arch_E