for short, this line of revolvers from Magnum Research
are truly the most powerful handguns in the world, with the exception of the Maadi Griffin
The BFR's method of becoming the most powerful production handgun is very simple, it does not fire handgun ammunition but rather uses big bore rifle ammunition.
Though quite reasonably priced (for what you get) these revolvers are kind of a niche product and is not very common. They are too big for concealed carry and really are too powerful for self defense, unless you are in bear country in which case the BFR is just right.
Calibers offered are the .22 Hornet, .444 Marlin, .450 Marlin, .454 Casull, .45/70 Government, and .480 Ruger/.475 Linebaugh.
Except for the .22 Hornet, firing any of these rounds in a BFR in your local range will most likely be followed by a blanket of awe and deafening silence.