Several years ago I bought a Crosby pump up air rifle for this purpose. However, “rifle” was a misnomer as it was a smooth bore, meaning that it wasn’t very accurate. Also, when pumped up enough to kill a squirrel it cracked about as loud as a .22 short. So, if discretion is your objective I’m not sure a high powered air gun is your solution. Might as well use .22 shorts, more accurate and efficient I would say. Suggest you check your local laws concerning shooting both firearms and air guns before you go forward with this endeavor though.
check the video here? http://www.youtube.com/watch?v=kBb7KReY6Eg