I hunt squirrels with air rifles every year. I've been playing with pcps for about 20yrs now. Fortunately now its a lot easier to get into the pcp realm. My son for Christmas just got a Umarex Notos .22cal. I put a red dot on it. Should be an awesome squirrel rifle.
Pcp air rifles are remarkably accurate and really it comes down to what kind of money you want to spend. They are by far the most accurate rifles in my safe. Air Arms S200 .22cal, Air Arms S510 .25cal, Taipan Veteran Long .25cal, and now the Umarex Notos. This being the most affordable out of the bunch. Have fun, they are addiciting. Check out Airgun Nation Forum