According to Robert Evans.
Movies treat the cocking of a gun like an exclamation point.
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. 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.
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.
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.
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.
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.
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.