Do not expect any air pistol to come near advertised speeds. Evanix, FX air guns and others make super powered pcp airguns that can reach into the 600-700 fps area, where as other spring pistols like the browning 800 mag and Diana p5 and lp8 can get around 500 fps.
The most powerful air pistol would be a Air Force Talon-p 25 Caliber. This is a PCP class air pistol. The air tank uses 3000 PSI to fire the pistol.
Considering PISTOLS ONLY, NOT rifles then: The Beeman P-1 is amongst one of the most powerful. But there are most likely others. But that is only in .177 caliber. Real power is in the PCP (PreCharged Pneumatic) air guns like the .25 Cal TalonP or the .22 caliber Marauder pistol. see the link below
Most likely it would be the Evanix Hunting Master AR6 by Evanix. It fires a 22 caliber pellet at 1000 FPS. This pistol is in a class of PCP pistols. (Pre Charged Pneumatics.)
A powerful pistol will have a powerful recoil.
Smith and Wesson 500 magnum revolver
The pistol in the movie Dirty Harry was a .44 mag - "the most powerful handgun in the world" according to "dirty" Harry Callahan.
First, it is rifle, not riffle. Second, no, or we would shoot tanks with a rifle. Cannon are more powerful. Finally, depends on the rifle. I have a .22 rifle and a .44 magnum pistol. The pistol is far more powerful than the rifle. I also have a .22 pistol, and a 30-06 rifle. The rifle is far more powerful than the pistol (in THAT example).
That would most likely be the Hatsan AT P2 PCP Air Pistol at 870 FPS
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It will shoot pellets or bb's. They normally use an air cartridge.
No way to answer- would depend on the pistol.