...Shouldn't a spark from a bullet start a combustion that travels much more rapidly than it would with the liquid leaking away from the main mass in the tank? In other words, go BOOM?
Lead doesn't spark. In other words, Hollywood got to you.
Not all "flaming planes" are shot in the fuel tank. I guarantee you fuel lines are somewhat modeled. Fuel lines can burn for minutes before "Catastrophic damage" is done.
It's less of a matter of 'sparking' and more of a matter of incendiary compounds and high explosives
The sport of understanding women.