Ahhh so your 16 years old ehh?
Well what you need to look up is Kinetic Energy first off. Basically Energy=Mass x Excelleration squared.
So to double your Energy you could double the speed or double the mass as I understand it.
A .177 cal pellet could have plenty of "killability" at 1k FPS, if its heavy enough. Most .22 cal pellet's kill squirrels in the 700-800 fps range. I use one to protect the bird feeder and around the yard.