What slugs do you use and what range would you effectively take a kill shot at for a squirrel. I know nothing about air rifles.
Also, I don't like the fact you have the finger on the trigger. That's called, bad trigger discipline. You should keep your finger off the trigger at all times, unless firing the gun of course, which you would not be doing inside, in the situation you are in here. Just a safety tip for you.
I used a distance of about 5 meters to shoot a squirrel.