If, for some reason, Bruce Willis needed to fight the devil atop an asteroid hurtling to Earth, he'd probably use a .44 Magnum.
For six straight days God gave birth to all of creation. And then, on the 7th day, he rested. He looked down at all the life he had created, and promptly decided that he needed something to kill all of it. So on the little known eighth day, Magday, he forged steel into chaos, and Mankind did rejoice, for he held in his hand the ultimate phallic symbol.
The .44 Magnum is actually the name of the bullet as well as the gun that fires it. Both of them are designed to work against anything with a heartbeat (even dragons, especially dragons,) because some psychotic sumbitch decided he really needed a more portable elephant-killer. For a long time, it languished in obscurity. Some say it was impractical, over-powered, and even ludicrous. The public was too scared to touch it, until one man stepped up to the rock and pulled out his Excalibur.
Enter Clint Eastwood.
Clint Eastwood was one of the first people to show the world that the .44 Magnum is meant to be used by men on other men (what? What's so funny? Jesus, get your mind out of the gutter: We're not talking about anything gay here; we're talking about long hard shafts and how they can best be used to penetrate other men.)
The .44 has an accuracy spread of only three inches at 50 yards. Since the chest of your average human being (or Terminator, depending on how awesome your hobbies are,) is substantially more than 3 inches, you just need to aim in something's general direction to make it go away in the most extravagant fashion possible.
Around 2003, somebody decided they needed something bigger than a .44, presumably so they could put bullet holes in time itself. They upped the size of the bullet, resulting in a cartridge so big that it can double as a strap-on - and oh, believe us, it will. Thus, the .500 Magnum.
The steel Thermos of death.
The .500 cartridge delivers 3,000 pound-feet of torque. Your average BMW has about 300 pound-feet of torque, so getting shot with a .500 Magnum is the equivalent of getting hit by 10 BMWs at the same time, in the same place. And since each bullet is the size of a fucking brick, that place is in the general vicinity of everywhere. As an added bonus, if the gun misfires you can just pick the bullet up and beat your target to death with it
While the .500 Magnum is rather appropriate for the primary purpose of killing the crap out of everything, most armed forces - military, police, Texans - don't use them. See, an integral part of a fire-fight is the ability to shoot fairly fast. If there's more than one target, or if you miss, most people like to try again, rather than shrugging their shoulders and just surrendering to death. With the Magnum, "fast" means about one shot per minute, during which time you're likely trying to put your shoulder back in its socket and pick pieces of whatever you just shot out of your eye. So maybe Dirty Harry wasn't the most efficient or practical policeman, but at least he was the most appropriately named policeman on the force: You do get rather filthy when every trigger-pull covers you in the bowels of the guy in front of you, and the contents of the bowels of everybody else within 50 yards.