Off the top of my head I can think of three *powerful* air pistols from my modest collection ...... at any rate, they came to me in UK legal spec very close to the 6 ft/lbs limit .......
BSA Scorpion ##
Weihrauch HW45
Sheridan EB
## I can speak from experience. The thing is basically a cut down BSA Meteor air rifle wrapped in plastic. Once whilst cack-handedly adjusting the trigger of my Scorpion, I managed to arrive at a *hair trigger* and narrowly missed shooting myself in the foot. It drove a .177 RWS HP clean through a maple floor board.
A good rule of thumb is the *HEINZ* test. If it can pierce both sides of a baked bean tin at ten metres then it is a good air pistol.
The BSA, the HW & the EB pass the test. I enjoy shooting the Sheridan ...... the trigger is ropey but it makes a very satisfying sound similar to a .22 rimfire. It reminds me of my old Ruger MK II