As this is a hunting site, we sometimes are the target of silly little anti-hunters that come here to stir the pot. As you are unknown and have asked what most here would consider an exceedingly odd question you must expect to be viewed with a jaundiced eye at first. Please keep this firmly in mind, and follow your own advice.

As to the original question, it depends upon the animal and where they are hit. A rat or squirrel is vastly tougher than a bird. At five feet almost any small game can be killed cleanly with even a slingshot - if the projectile strikes a vital area. A light weight and low velocity projectile would work but the impact point would be critical. If a vital area is not hit and the animal is not taken cleanly then it is just cruel. Cruel behavior is not looked upon kindly by hunters.