A good point Red Dragon; the .50AE is a whole lot different from the .50BMG round (used in the M2 Browning Machine Gun and some specialized heavy target rifles). The .50AE makes for a huge, heavy, and almost unmanageable handgun. The .50BMG round is much bigger and heavier, and is way more powerful; trying to design a .50BMG Desert Eagle or .50BMG revolver would make for a handgun that would almost demand its own tripod.
But this is a digression. The point is that in response to the OP’s question 2 :" Couldn’t a handgun be made that fires the same bullet as a high-powered rifle, resulting in the same muzzle velocity and therefore the same “power” (energy)?" The answer is no, not really. The closest you could come would be to have a very cut-down rifle (essentially a rifle reciever with a pistol grip and short-short barrel) that in the eyes of some would pass for a “handgun”, but even then, it wouls still get less MV and thus “energy” than the full rifle. A true handgun redesigned for rifel rounds --a mammoth revolver or an even more ridiculously engarged autoloader than the Desert Eagles— would be unuseable.