A few points on this one.
"Expansion" in a sub 12 ft.lb airgun pellet when striking anything softer than a steel plate is a myth. I've dug hundreds of pellets out of various animals, and have never seen one that has expanded. No matter how hard/soft the lead is.
The amount of retained energy is of little importance. What matters is placement. As long as the pellet has sufficient energy to penetrate to the depth of the vital organs (to penetrate the "hardest" point on a rabbit, the skull, requires around two foot pounds) then placement is the prime factor. The difference between a .177 and a .22 in terms of "terminal ballistics" is entirely negligible.
"Knock-down" or "stopping" power is a myth. We all know that a .45 slug, when it hits you, will send you flying backwards right? Wrong! The laws of motion (specifically Newtons second law of motion) tell us that for each action there is an equal but opposite reaction. So if the bullet could cause you to fly backwards, then of necessity the firer would also fly backwards. The only "small arm" I've ever seen achieve this are the elephant guns, .600 Nitro express etc.
A .22 or .177 air rifle pellet will NOT cause a huge amount of damage. Forget "wound track" and "pellet expansion" and "hydraulic shock". An air rifle pellet causes death by one of two modes.
Either the pellet strikes in the brain/spinal column, causing damage to the structures which control the respiratory systems and causing immediate cessation of oxygen delivery to vital organs and muscles. Muscles might keep going for a few seconds on reserves of oxygen in the blood, but not for long, and the brain will shut down almost immediately.
The second mode is to cause death by lack of oxygen by destroying the heart and/or lungs, or another blood laden organ. The amount of time it takes to die depends on the rate of blood loss, and the individual animals tolerance to blood loss. If you hit the heart, and damage it sufficiently to stop blood flowing, then death will be rapid. A hit in the lungs that misses the heart will cause blood loss and death, but the animal might be able to get a fair distance before death. Hit another blood laden organ (liver, gut, major artery) then death WILL occur, but it can take minutes, or even hours, for sufficient blood loss to occur. Not an acceptable occurrence in air rifle hunting.
So the moral is, at sensible air rifle hunting ranges, there is little to no difference in the killing ability of the .177, the .20, the .22 or the .25. What is more important is the ability to put the pellet in the right place. With work, you can develop the skills to do this with whichever pellet you choose.
There are some good articles on the net about knockdown effect, and the relative power of different ammunition. Granted they mainly refer to humans, and large calibre handgun ammunition, but the principles are the same.
This has some good info, and some good links to articles:-gunshot wounding
Edited to add, if you don't believe me, do you believe the FBI?FBI firearms research paper "Handgun wounding factors and effectiveness"
Edited by matt_hooks, 28 February 2011 - 12:16 pm.