How hard to hit things with an airgun might not seem like a survival problem until you need to put meat on the table.
Generally speaking the experts state that it takes a minimum of :-
2 foot pounds to kill a bird and
5 foot pounds to kill small game. i.e. Rabbit or Squirrel.
As for human injury, it only takes between 2 to 3 ft lbs to penetrate the eye globe. A VERY good reason for wearing shooting glasses.
How to calculate a Foot Pound of Energy(Pellet Weight grains *fps*fps) / 450240
1 gram = 15.43 grains
100 fps = 30.48 m/s