RE: Slowest FPS but still able to kill
I guess what Howler is describing is Energy, not velocity. Energy = (mv^2)/2. One half mass times the velocity squared.
The rock is the big mass, with slow velocity.
The pellet is the small mass with the big velocity.
The real secret in arms development is the square of the velocity. Barrel damage and shot drop is a function of the velocity, but energy is the function of the velocity squared, which increases much faster than the velocity. Hence the popularity of the 223 and the .17 rounds.
You could that bird with a feather, if you could get it going fast enough.
PS: I like Colibiri 22s, but just because I don't own an air rifle accurate enough.