I was amused to come across this video from the well-known Forgotten Weapons channel. If it's not the largest-diameter cartridge available in a revolver, it'll do until someone with more money than sense makes a bigger one!
It's not the biggest-bore handgun I've ever seen, though. That honor goes to a percussion-fired muzzle-loading 10ga. shotgun, its barrels cut down to 10" and its stock converted into a pistol grip. It belonged to a farmer in South Africa, who said he'd had to deal with one too many marauding predators on his sheep farm. He reckoned that, at ranges out to 15 yards or so, he'd put two full ounces of buckshot into one of them, and follow it up with a second dose if needed. He said that was seldom necessary - and I believe him! He offered me the chance to fire this handheld behemoth, but I declined, as gracefully as possible. I could feel the carpal tunnel syndrome coming on, just by looking at it. (He was about 6' 4" tall, and broad as the offside of a barn - all muscle, too - so the recoil didn't seem to bother him.)
I'd like to fire that .577. With blackpowder propellant, it wouldn't kick too hard, methinks; and the cloud of fragrant smoke blowing downrange should be an interesting sight.