An air rifle for hunting use needs to have knock down power, no doubt about it.  A .177 caliber hunting air rifle doesn't have much power, but can still be useful in dispelling small game and can be used for squirrel hunting, we would recommend head shots! The best air rifle for small game is going to be a .22 caliber air rifle, the pellet will be large enough to kill, and it won't be traveling so fast to the point where it just passes through the animal as a .177 pellet rifle would.  

A hunting air rifle has its advantages over a traditional firearm which uses gun powder.  For one, they are quieter and can be used in your backyard without your nosy neighbors coming to check what all the ruckus is.  They can be purchased without additional licensing, and they are not nearly as dangerous to you or others as a traditional rifle is.