OK, let’s say that I know this guy who is in a pitched gunfight with a group of bad guys, and he notices that they are standing next to a propane tank which has presumably slipped their attention. What will happen if he shoots it? Big explosion? Gas leak? Relatively harmless ricochet?
Need answer fast. 
Pistol-9mm, ricochet.
Rifle, gas leak.
Mythbusters did this one, if I remember, they needed a minigun to get a big boom.
Depends on your aim. Propane tanks are cylinders. The higher (or lower) you hit, the greater the angle, the more likely the deflection. If you want to punch a hole in it, you need a relatively centered shot and a pretty good-sized round.
If you want an exciting explosion, you’d better be shooting tracer rounds.
What about once I make the gas leak, and some foolish mook on the other side pulls the trigger again? Will his weapon firing generate a spark that could ignite a propane leak?
The Mythbusters episode mentioned above made it look like propane was scarcely more flammable than cooking sherry. They had to try pretty hard to even get as much as a jet of flame out of the tank, and it took something not much smaller than a rocket-propelled grenade and some serious monkeying around with the safety devices in a standard barbecue tank to actually make the thing explode.
Only if it happens to be at the appropriate oxygen/propane mix. The thing with sparks and gas explosions is they don’t just happen, you need the right mixture for anything to ignite. In an open area this rarely happens and in a closed are you need good timing because to much or too little propane and you don’t get the boom.
You have to have flame around it for anything interesting to happen.
We’ve done this. One Thanksgiving a few years back, we soaked a patch of desert with lighter fluid and parked a propane canister in the middle. Lit the sucker, backed off a ways, then shot it with a .223. Small flame, BIG flame, and the cylinder went flying about a hundred yards. But it was intact when we found it, with just a couple of holes in it from the bullet.
Most things we use as fuel aren’t going to be super dangerous.
If propane and gasoline exploded as easily as the movies and video games show it’s unlikely they would be used like they are or stored like they are (any Joe can go down and fill a gasoline container at a pump.) In real life you can take a high powered rifle and shoot the giant 150-200 gallon fuel tanks on a semi-truck and it will just mean someone is out a lot of money replacing the fuel and fixing the tank, but nothing is going to blow up.
Another way of thinking about these sort of things is imagine you have a gas can filled with gasoline. You take a nail to it and a hammer, and drive the nail into the tank.
No one realistically thinks that would cause an explosion, it would just cause a leak in the can.
Propane isn’t any different really, the only difference is a propane tank is pressurized. So if you were able to puncture a whole in it with some sort of hand tool the danger would be the rapid escape of the gas and such, but the chances of explosion aren’t much different than when you drove a nail into a plastic gasoline tank. Propane and gasoline are not designed to explode from percussion. If they were they would be very, very dangerous to transport, and you could expect a propane tank to explode if it fell off a truck and rolled onto the interstate.
Did you mean shoot AT a propane tank? Or shoot a propane tank OUT of some kind of gun?
Now that’s going on the ol’ Xmas list this year.
Does it matter if you hit in the vapor on top or down in the liquid? At least with gasoline, I think a hit up in the vapor/air mix would be more likely to ignite. Propane tanks would be all vapor and no air on top.
Propane is plenty flammable. Just not without oxygen present.
The science is called Stoichiometry and it deals with fuel/oxygen ratios necessary for combustion to occur.
You must have oxygen (or a few other gasses but mainly oxygen is the thing) to allow combustion to happen. Too much or too little will either diminish the burn rate or make it impossible completely. Different gasses have different ideal ratios.
This is why when you light a gas stove the fire does not burn in reverse down the gas line (no oxygen in the gas line). That is a good thing.
When you shoot the tank you poke a hole in it but it happens far too fast to mix the air and the gas to a point where it will ignite. Only when they went to extreme lengths and were able to disperse the gas rapidly into the air could they get an explosion.
ETA: I’d have liked to see them shoot the tank once, wait differing amounts of time to allow the gas to bleed out then shoot it again. I suspect if conditions were right (no wind blowing the gas away) the second shot might produce an explosion.
Not to spoil anything plot-wise, but there is a really great optional scene in Mass Effect that works that way. Bad guy du jour is monologing, Shepherd interrupts with “You talk too much.” and shoots the gas line. Bad guy stops for a moment to consider what just happened, and then Shepherd shoots again to ignite the gas.
Mythbusters did that one, too. Sorta.
They were trying to ignite a room full of gas with the muzzle flash.
They had a hell of a time getting the ratios right. (Stoichiometry as mentioned by Whack-A-Mole)
Hunter S. Thompson did this regularly for recreational purposes. I know that somewhere I have read a description written by him of what it looked like (e.g. a brilliant flash of fire lighting up the night", etc.) but I can’t find it.
I do remember it sometimes involved attaching a blasting cap or some additional explosive to the propane tank before shooting it. But other cases seem to have “blown up real good” with just a tank and a shotgun. I can find plenty of references to him doing it, but most of them drift off into a story like thisbefore getting to the specifics of the fireball it created.
Disclaimer:
DO NOT try this at home (or anywhere). The science is clear on this but nevertheless playing with flammable gasses in this manner is a recipe for disaster.
If you shoot the tank the gas will bleed out. Once mixed with the air there is a great potential for a serious explosion. Fuel-Air Explosives (FAE) are among the most devastating conventional bombs in a military arsenal. Poking holes in a propane tank/shooting it is a recipe for killing a bunch of people.
That is sage advice regarding almost anything that Hunter S. Thompson did for recreational purposes.
Indeed. He’s one of my heroes for doing this:
That sounds more dangerous than shooting propane tanks.