40s are very popular here because of the law. 40 and under for small game and 40 and over for big game. Of coarse they will tear a squirrel or rabbit up a bit more. But they shine on bigger varmints like groundhogs, fox and coyote. With a rabbit you need to head shoot them and it will pretty well blow their head off. For squirrel you can rib shoot them and really not ruin much, no meat there anyhow. The heads get saved for the old timers to make dumplings with so that's a no no to head shoot a squirrel here.
For deer it's a little light and I hear some stories every year of folks wounding them. I do hunt deer with a 40 from time to time but I use a 175 .401 swc cast in pure lead. Up close it has as much power as a 50 prb. Out further it has more power than a 50 because bullets hold more energy down range than a ball does with their better ballistic coefficient.