Maybe you don't have to actually kill the squirrels. I have fruit trees in my back yard and over the years have learned that a squirrel whose territory includes your house, who understands not to go near your house, will help you keep neighboring squirrels away. Shooting them just brings more newcomers a couple weeks later. Currently I harass the resident squirrels, with my dog's help, but let them live to squabble over territory with neighboring squirrels. An airgun can help with that as you can reach out and touch the squirrel even if it is far away, and if it is up in a tree where the dog gets frustrated. In my backyard, with the longest distance of ~45 yards, a multipump gun that shoots BBs works best. More distance = more pumps, closer range is safer with less pumps. BBs are relatively safe because they shed power and speed very quickly, and if your yard is small then airsoft BBs might be even better than metal BBs.
That Crosman 1322 has a sister 1377 that will shoot regular BBs, and isn't hard to pump a few times, which is all you'll need to harrass but not kill. Glass windows should be safe at 1 pump beyond 10 yards, 2 pumps 15 yards, 3 pumps 25, 4/35, 5/40, 6/45, 7/50, 8/55, which is about the distances you'd harrass. At those distances with that many pumps the BB won't break the skin. The pumping action of those is an accordion motion, just look for a Youtube video for whatever airgun.
Plus if you just only shoot to harrass then you'll have more squirrels to practice on tomorrow! If you kill them then you and your dog have to wait a couple weeks for more to show up.
