Douglas' Gun
So..why was Kirk Douglas carrying a Derringer? Seemed kind of dumb for a cowboy
shareHe explained it in the film. He said it had a larger caliber than the Colt .45 and no handgun was good past 20 feet, so a larger caliber would make it better than a Colt.
Now, if any of that is true, I've no idea! But that's what the character said.
Eddie Valiant: Nice booby trap. RIP, Bob.
Perhaps we need to have a gun expert weigh in (and I'm sure there are plenty out there).
His preference did sort of puzzle me too. This "oddity" seemed to be another aspect of this character's rather eccentric nature.
I've owned many derringers in my life, from .22 magnum through .45 caliber (and several Peacemaker replicas), and I've never known of any derringer greater than .45 caliber. And in the 1800's, there certainly wouldn't have been any greater than .44 or .45. Of course, you could custom-load cartridges to be more powerful than standard loads (assuming the derringer could safely handle it), but you could do that for six-guns, too.
As for six-guns not being accurate beyond 20': well, there's no magic number for that - 20', or any other. There's a simple rule of thumb for ANY gun: you'll have better accuracy close-up, and increasingly worse accuracy the farther away the target. The main factor is the length of the barrel - and, correspondingly, the sight radius, i.e. the distance between the rear and front sights atop the barrel. A Colt Peacemaker's barrel (ahead of the chamber) was typically 7 1/2" long, while the barrel of a derringer like Kirk Douglas' couldn't have been more than about 2" long beyond the chamber.
In addition to better aim provided by a longer barrel and its longer sight radius, a longer barrel means a more powerful impact by the bullet. A 7 1/2" barrel has 5 1/2" longer for the bullet to be pushed out by the exploding gunpowder's gasses than a 2" barrel, resulting in a substantially faster (thus, more powerfully) traveling bullet out of the six-gun than out of the derringer, even if the cartridges are identical.
In addition, there's the matter of firepower. A Peacemaker has six shots, and a derringer only two. And while they're both probably just as cumbersome to reload, with a derringer you have to reload much more frequently. In a gunfight, that's an issue.
So, even if Kirk Douglas' character O'Malley were more comfortable with his preferred derringer in close quarters, it would be rediculous to suppose that the derringer is in any way superior to the six-gun. It's less accurate, less powerful, and, being much smaller and lighter than a six-gun, much harder to handle and control firing the same cartridge.