No its not based on what a spring piston airgun can do , its about on what airgun quarry needs . A 22 Lr can shoot a lot better a lot further again , 22CF like my .22 hornet or 223 rem can go still further ,its definitely all about what energy the airgun does not produce that makes it most useful to myself , not what it can ! To my mind , I dont think I have not had at least one air rifle in my possession for something like 45 years now ?Is that 20 ft lb limit based on the max power a springer can comfortably produce ? It seems a little arbitrary. It puts a .22 pellet trajectory somewhere below that of a sub 12 .177
I don't think exact FAC power is that important in FAC air rifles. Very few airgun quarry will benefit from the additional energy (except perhaps fox). But you may as well have the flattest trajectory you can- with whatever pellets suit the rifle. Something around 950fps seems a rough limit for pellets. Slugs can approach 1050 I believe.
So for .22 somewhere around 30-35ft lbs gives you the flattest trajectory possible.
When you can clear out a whole field of rabbits with a CF its hard returning to long careful stalks taking 35 yard shots



