I think large, heavy, expensive, custom made double barreled rifle. To the best of my knowledge, there was one handgun made in .600 Nitro Express, more as a curiousity than a usable handgun.
No, the .600 Nitro Express is more powerful. Also, there is a .700 Nitro Express round that is designed for a rifle, but there is one handgun that will fire it.
Probably, almost certainly, the .600 Nitro Express is the overall most power handgun bullet. Let me alter this answer by saying the .600 Nitro Express is a rifle round that one company made a pistol for. The most powerful handgun cartridge is the 500 S&W Magnum.
a .700 nitro express
There's the .700 Nitro Express.
Not the first double rifle (they have been around since muzzle loading days) but the first "Nitro Express" was created by Rigby in 1898 as the .450 Nitro Express. This used the newly created cordite propellant in lieu of black powder (that is the nitro part of the name). There was soon after several variants and other companies created their own versions.
There has been a revolver made in caliber .600 Nitro Express. This was an elephant rifle cartridge. As a firearm, it is a curiosity, and not a practical shooting firearm. There are also unconfirmed reports of a Maadi bolt action pistol in caliber .50 BMG.
Possibly the .460 Weatherby Magnum, the .577 T-Rex, or the .700 Nitro Express.
the most popular right now seems to be 470 nitro express in a double rifle.another is the 416 rigby,with the 458 Winchester another candidate.
Compared to a 22 short, probably. Compared to, say, a .500 Nitro Express, not at all.
"Cheap" and "700 Nitro" should not be in the same sentence. Last one I saw sold, with case and accesories, was $69,000. Before that, $79,000.
The Rhino rifle is chambered in .600 Nitro Express. The loading specified for this round is 120 grains of cordite.
The .950 JDJ is the biggest rifle The biggest hand held gun is the 600 Nitro Express Pistol