A .22 is a pretty powerful projectile weapon. I would take it over a bow or crossbow any day.
Yes, I agree. A single well placed .22 rimfire rifle round to the head would probably be fatal.
Here is a pellet that is heavy. I am sure the fps would drop but the impact would be more:
Beeman Kodiak Double Gold Pellets
.22 Caliber 21.14 GR 200 ct
http://www.airgunsbbguns.com/22_caliber_pellets_air_guns_s/115.htm
http://www.airgunsbbguns.com/v/vspfiles/photos/bee3250DG-1.jpg