Once upon a time, I owned a Thompson/Center Contender with an SSK Custom barrel in .45-70 rifle caliber. That was a handful, and when it went off, stuff downrange moved.
I also owned a short-barreled version of the North American Arms mini-revolver, in .22 Magnum caliber. It was quite a handful for such a tiny weapon, and unless you wanted the hammer spur buried in the web of your hand after firing, you had to hold on to that thing with the same grip you'd use to strangle a rabid weasel.
But the wildest hanggun I ever shot or owned was a Pachmayr Dominator conversion kit in .308 Winchester rifle caliber. It bolted on to any 1911A1 pistol frame, converting it to a bolt-action single-shot hunting pistol. If you are one of the many folks who think the 1911 isn't a particularly comfortable gun to shoot in its original caliber/configuration, then the Dominator would not be a good choice for you. After a few shots, the grip safety would start my hand bleeding at the web, and I'd have to stop and stick a bandage on it. In low light, the flash was spectacular, and the boom would draw folks from the far end of any firing line to see what the fuss was about. My favorite load was tracers, fired at 100/200 yard steel targets. Lotsa fun!
This one is not mine, just a couple of photos of one I found online: