re: Air rifle or subsonic .22 for backyard squirrels?

People ignore or disregard the air rifle option, but those are great. My brother has taken to them to eliminate small pests (mice mostly) around his house, in a BR subdivision. If you get the springloaded style, that requires you cock the barrel, you will get plenty enough power to take down little varmints, and it doesn't cost too much more to get a suppressed model.
The noise is about like a flyswatter, maybe a bit more, but definitely not gunshot decibels. (Note- with a non suppressed model, the first couple of rounds will "diesel", or ignite any oil in the barrel, and it will sound like a 22lr rifle. And smoke, too!)

He's got a pretty cheap scope mounted and has zeroed in so that he can hit a dime at 25 yds or so. You will need to buy a scope designed for air rifles, because the spring has a jolting recoil, will damage the optics of a regular scope.

These are so effective, so cheap to run, that he's since upgraded to a Benjamin canister model in 22 caliber. He's one-shot dropped a raccoon trying to dig into the garbage cans with that. The canister models cost more, but since they eliminate the spring issue, you lose the harsh recoil, and you get maybe 5-10 shots before having to pump it back up. These are way more powerful than your childhood pump pellet guns, we're definitely into powder territory here.