i've shot squirrels with my pellet gun. along with a chipmunk or two, some moles, and a few birds. my gun is reletivly slow, shoots 640fps, but it was still able to do the job on squirrels. i used hollow points and had 'em traveling straight through squirrels heads. instant kills from about 25 yards off of trees. moles are fun to shoot with my gun to. i'll take it out and scan some brushy area waiting for one to come out and feast on some grass. once one does i use my open sites to nail 'em. they're hard to hit at 20 yards with open sights lemme tell ya. i have made some nice head shots on them though.
if your new to using airguns for small game then drop me a PM, i might be able to give ya some helpful hints. otherwise i'll see ya around the forums thunter