More important than velocity ratings and caliber is the ability of the shooter. All the power in the world won't do a bit of good if the guy holding the gun can't shoot it straight. A .177 making 750 fps(9.8fpe) at the muzzle has enough power to take a squirrel at 40 yards if the right pellet is used. All you need to do is hit it in the right spot-if you can. Power, velocity and caliber shouldn't be used as a substitute for ability, nor should they used as interchangeable terms. Power is the end result of pellet weight and velocity. Velocity itself does not make power, there must be sufficient mass along with it. Caliber size is not a guarantee of power either without sufficient velocity. Low velocity large caliber guns may not make any more power than high velocity small caliber guns. Either way, a gun shooting at 10 fpe is more than enough for any small game such as squirrels, crows, etc. In the example cited above, hunting turkeys with an air rifle, the pellet doesn't need to penetrate deep if you're doing it right. The hunter needs to be able to make a head shot. Realistically thats the only way you're going to drop a 20+lb bird with an air rifle. So whats required isn't neccessarily a very powerful large caliber air rifle. Any rifle of sufficient accuracy and even moderate power, in any caliber, will do the job as long as the shooter is capable of doing his part.
