I have shot lots of rats, ( the big 12- 18" long pack rats) with my 10-22 with cci std vel and win t-22 with great results. They did not seem to have too much over penetration. I have also used a daisy(I think?) single pump (barrel breaks) pellet rifle too. It puts them out at about 800 fps I think, and it's .177 . I've never had any problem with head shots with it at 20 feet or so. Any thing inside a house or shed.
A little more on the fun side: My brother and a friend and I were exploring an old mine shaft in our area once and found a family of pack rats living about 40' inside the entrance. Desperately wanting to prove our superiority over these vile creatures, yet not exactly wanting to start discharging firearms inside a 70 year old mine that had been abandoned for 50 of those years, we were at a loss until I remembered our paint ball guns.
4 lb pack rats don't seem to like .68 cal. projectiles traveling at 300 fps.
In fact, they did not survive the whole little experience. Then, on our way out, this poor, obviously suicidal, bat starts flying around like it thought it would freak us out. Why would we be freaked out standing in a shaft six feet wide by seven feet high with just a couple flash lights for light and a bat flying around our heads?!?!
No, we just very calmly, in a very cool manner began to empty our paint ball guns as fast as we could. My friend actually did hit the flying bat! It died immediatly. (only took a couple hundred rounds!) We then went home feeling very manly indeed!