Where You've Seen It:
Boondock Saints, Die Hard, Reservoir Dogs, all the movies on this list.
Movies treat the cocking of a gun like an exclamation point. When Hardass McBadCop interrogates the lone surviving henchman, you can safely assume that, at some point, he's going to make his gun go "clickety-clack" to let the poor schmuck know he means business. The sound of a clicking gun is so ominous that the MacManus brothers use it to close out their fruity little prayer in Boondock Saints:
That "click" is the sound of a hammer being cocked back, and movies seem to be saying, "This means the gun is ready to fire now, baby!" It doesn't mean that, however. It doesn't mean anything. The gun was already good to go.
For instance, the guns our hunky Irish assassins are using up there (the Beretta 92F--the same gun John McClane uses in Die Hard), is made so that pulling the trigger also cocks the hammer for you, to save you the extra step and the extra two seconds during which you could get shot. The "cocking the gun to show you mean business" must date back to Westerns, back when those old revolvers forced you to cock them between each shot (something that was made obsolete 150 years ago--so, yeah, Hollywood is even slower to catch up with gun technology than they are computers).
They've hacked your car!
By the way, when you fire one of these guns like the Saints have up there, it's made so that it leaves the hammer cocked back in between shots (the reason is it makes the trigger a little easier to pull). We bring this up because that means the MacManus brothers purposefully de-cocked their guns before shooting that mobster, just so they could make that sound.
It gets sillier. When movies show somebody with a gun that doesn't have a hammer back there to be cocked (like a shotgun or assault rifle) they substitute either the pumping of the shotgun or pulling back the slide on the automatic. It's the only way to get a cool clicking sound for dramatic effect.
The problem is that on these guns, that only serves the purpose of ejecting an empty shell and sliding a new bullet into the chamber--something that already happened the last time you fired it. So every cool "click" would be accompanied by the somewhat-less cool sound of one of your perfectly good bullets falling to the floor.
Where You've Seen It:
Jaws, Casino Royale, Matrix Reloaded.
In the movies, bullets and anything mildly flammable have a matter/anti-matter relationship. The second hot lead touches a car's gas tank, it and everyone inside are going up in flames. This is incredibly convenient for those times when Morpheus needs to flash-fry two creepy dreadlocked albinos or a Buick full of raw bacon.
We bet crisp, bleached dreadlocks taste like Sun Chips.
Propane, hydrogen and oxygen work the same way. As long as it is packed in a pressurized metal cylinder, you can be sure shooting it will result in an explosion large enough to blow through any jam the screenwriter gets the protagonist into. Shoot an oxygen tank in a shark's mouth and he'll blow like he's spent all week munching on dynamite.
The manufacturers of automobiles and pressurized containers really don't like liability lawsuits. If their products could be turned into a fireball the size of a city block with nothing more than a sudden impact or puncture, every car accident would look like the Fourth of July, every pile-up would look like a Michael Bay movie.
The Mythbusters famously demonstrated the falsehood of both the "shoot the gas tank" myth and a ton of other gun myths in two of their episodes. As it turns out, you actually have to coax a car into exploding by doing things in a very particular way. If you can punch a small hole in the tank, light a fire outside of it, and vaporize the gas inside to the point that the tank over-pressurizes, then you could probably get it to light. Assuming you use special tracer bullets.
Not those kind.
What's so illogical about Hollywood's "handguns can explode a car" principle is that their bullets can't penetrate anything else. Here's John Cusack hiding behind a shelf of potato chips at a convenience store, safe from the dozens of bullets slamming into them:
And if the good guy takes cover behind a car door? Hell, he might as well be holed up in Fort Knox. Ironically, while guns are useless for exploding a gas tank, they'll punch through a car door with ease.
Everything we know is wrong. If those of us raised on action movies have to fend off a Red Dawn-style invasion, it's going to be a total clusterfuck.
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For more ways Hollywood will get you killed, check out 6 Life Saving Techniques From the Movies (That Can Kill You) and 6 Horrible Lessons Hollywood Loves to Teach Kids.
And stop by our Top Picks (Updated 06.14.10) to see the Matrix-style shootout the columnists have on their lunchbreak.
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It's loaded with facts about history, your body, and the world around you that your teachers didn't want you to know. And as a bonus? We'll also explain why everything you know about explosions is wrong.