i used to have an old BSA 'mercury' in .177. could happily shoot pidgeons and rats from reasonable range.I recently aquired another .177 with a scope, and i really prefer iron sights. A good friend of mine has polish family and he has some fantasticly painfull .22s from poland. (yes, we shot each other from 60 yrds to see whos gun was more powerfull)
some airguns ive seen are capable of penetrating your flesh from close ranges, but most i have used wouldnt kill a baby if you put the barrel up its nose and fired, research well or you might waste alot of money.