Frank James, writing on farm guns, finds a surprising winner. Interestingly, I once sold one to a guy who planned to use it for the same thing.
Another customer once had a Springfield M6. Deciding that .22LR wasn't enough, but .22 Hornet was too much, he had us take a chamber reamer and convert his to .22 WMR. It was from him that I learned that CCI's .22 Magnum TNT round was not a good choice for squirrel hunting if you actually planned to eat the tree rat. Apparently at ~50 yards or so, it will scatter squirrel all over the forest.