I see this a lot in movies. Have anyone studied whether this is easily doable?
The Discovery Channel TV show MythBusters tackled this question in a 2004 episode, shooting five bullets into the gas tank of a Cadillac. There was no explosion, not even a fire. This raises the question of how a bullet would cause an explosion in the first place. To ignite gasoline, both heat and oxygen are required. If there's insufficient air in the tank, an explosion won't be possible, though an empty tank would be more likely to explode than a full one. And while a bullet is heated by the explosion that propels it, the bullet itself won't necessarily be hot enough to ignite gasoline.
I remember seeing the episode, and they seemed to do a reasonable job testing it, using various fuel/air mixtures to see if they could get it to work.
They also tried tracer rounds in a "Mythbuster's Revisited" episode, and also didn't manage to make it explode:
It has already been proven that when shot by a normal bullet a gasoline tank will not explode. However, if a gasoline tank is shot by a tracer round from a great enough distance so that the round can ignite with air friction, it will cause the gasoline to catch fire. By the time this happened the tank was so riddled with bullets (from previous tracers that were fired too close to ignite) that there was no contained pressure, but the MythBusters surmised that had the tank been properly enclosed, it may have exploded; but overall it remains extremely improbable.
So, it's possible to ignite gasoline with tracer rounds, but really hard to get the tank to explode, since it loses pressure too fast.
That doesn't mean it's impossible, but the Mythbusters (who specialize in blowing things up) weren't able to do it on purpose, so it's definitely not "easily doable".
The Insultingly Stupid Movie Physics also did an analysis of this trope:
Can a Handgun Bullet to the Gas Tank Blow up a Car?
The mixture in a gas tank is typically too rich to explode and this alone precludes the oft repeated shoot-the-gas-tank-blow-up-the-car scene. Furthermore, copper jacketed lead bullets are not good spark producers, hence, not good igniters but what about a steel jacketed or steel core bullet hitting that one in a million gas tank that is so close to empty it does have an explosive mixture in it?
When a piece of steel is ground, it emits a shower of yellow orange sparks--tiny particles of superheated metal. Judging from the yellow orange color the particles are over 1000 °C, certainly above the auto ignition temperature of gasoline air mixtures.
We donned safety gear and poured a small quantity of gasoline in an aluminum pie pan—just enough to wet the bottom. Using a pneumatic grinder on a bolt we showed [sic] the pan with sparks. The result: nothing. We dropped in a lit match and poof—flame.
Okay, no number of experiments can ever prove that grinding sparks will never ignite gasoline. There's no way to test all possible conditions. Grinding near gasoline is dangerous, but it's also clear that grinding sparks aren't a reliable source of ignition.
We didn't test bullets, but it's doubtful that one, particularly a handgun bullet, will reliably set off gasoline fumes.
The above experiments were conducted in a safety conscious manner under the supervision of a qualified professional. Do not attempt them on your own.