I hunted with powder burners for 40+ years before killing something with an air rifle. I had a blue streak most of this time but when I went hunting I took a shotgun or rimfire instead.
Two years ago I bought a little 1377 and shot a squirrel with it that was chewing on my gutter. I was using the stock peep sight and hit it square on the shoulder. It broke the shoulder but did not penetrate into the body. The squirrel died but it took several days - I found it later in the yard. So the 1377 got retired from squirrel duty. It finished off a bird that flew into a window of my house, however, and killed a couple snakes.
Last year I bought a Prod and have enjoyed using it to knock squirrels out of my trees. I found the stock 13fpe or so to not be totally reliable in knocking them down without running so I tuned it up to about 18 fpe and haven't had issues. Shots are 15-30 yards. Head shots worked fine at 13 but when I miss the head and hit the vitals, the extra energy results in nice clean no running kills. Only one I lost was a brain shot that flopped into a hole in the root of a tree that was too deep for me to drag it back out. I tested penetration on dead squirrels and at near stock power levels my Prod would not shoot through a squirrel side to side at 25 yards. I tried Crosman 14.3, H&N FTT, Barracudas and metal mags. At 18 it does. If I happen to shoot the length of the squirrel it does not exit but the couple that I've hit this way have still dropped on the spot. I find the pellet under the hide so it goes the length of the squirrels body. Does a lot of damage.
My newer Avenger (25) has only one squirrel on its account but will clearly get it done. The Prod has taken 13 so far. I keep hoping the muskrat that likes to chew on my dock floats will show his sorry head but no luck on that so far.