You can kill a deer with a 22. A 40 cal in my opinion is a bad choice.

My partner had to put down a deer the other day with a 40 cal. Muley was already hopping around on a broken leg. Bullet to the head from 15 yards slowed it down enough to get a clean headshot.

Its a decent round for a defensive weapon. Not for a clean kill on moderate sized game though. It was never designed with hunting in mind.