Not even close. There are several more powerful handgun cartridges, not counting the single shot pistols that use rifle cartridges. While Dirty Harry may have proclaimed it to be "The most powerful handgun in the world"- it is not.
600 Nitro Express
Factory, I'd probably say the 500 Magnum
M2 can be hand held, but, that is the exception. MP5 in 10mm is along the lines of what you might be asking about.
Other than a toy gun or water gun, the next most powerful one a citizen can have without a permit is a "B.B. gun".
yes but it wont be as powerful
577 T Rex
star cannon
The magnum .55 is the most powerful, but some Desert Eagle has the highest calibre bullet.
10 gauge shotgun
Probably the siege gun.
It is the dragon destroyer.
The .500 S&W Magnum is the most powerful handgun cartridge in production. Smith & Wesson Model 500, most powerful revolver. There are other handguns that are larger but are not in production. One off customs etc.