Bullets Explode Everything
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.
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.
If you grew up watching action movies, you probably believe all these. Well, at least now you know the truth.