Q:

How vulnerable could space launch vehicles be to a "lone gunman"?

As the saying goes - space is hard. Rockets are a collection of extremes in temperature, pressure, and energy packed as closely as possible and driven to their limits. Even in 2016 they still occasionally fail catastrophically. For example in todays Los Angeles Times business section:

Bill Ostrove, an aerospace and defense analyst at Forecast International, said SpaceX's reliability with the Falcon 9 is 93%, which is "right in the ballpark" of the industry average of 95%.

While most transportation has a per-trip reliability of four, five, or even six nines (i.ie 99.9999%) putting things into orbit doesn't quite have even two nines yet.
But I couldn't help wanting to at least consider if this event could have some non-rocket-related explanation. So I thought I would hypothetically ask out of morbid curiosity, could this event have been caused by some malicious human intent. How vulnerable are space launch vehicles to wackos?
The Mythbusters showed that you can't just shoot a car's gasoline tank and make a Hollywood-friendly event:

Explanation: In Hollywood, it seems like all you need to blow a car to smithereens is a gun and a gas tank. A garage-full of flicks depict people destroying automobiles by shooting straight into to the tank, triggering a massive explosion.
But when MythBuster Jamie Hyneman bored into a Cadillac's full gas tank with a barrage of bullets, no such detonation went down. His six slugs simply ripped right through the tank and out the other side of the otherwise intact sedan.
With no flames to be found, Jamie's gas tank target practice busted the cinematic motor myth.

Citing an entertainment TV show like Mythbusters for a source of factual engineering information is a little painful in itself, however in this case it looks like fairly reliable results.
edit 1: A rocket filled with fuel and oxidizer and various other pressurized gasses is not identical to a car (hat tip to @PeterCordes for pointing that out for us) and cars can not be extrapolated to rockets. So I have asked:
Could a few high-velocity rounds have released enough RP-1 and LOX to have initiated the chain of events seen in today's fast fire (not actually an explosion)? I am not looking for speculation, or opinions of the likelihood. From an engineering perspective, could a few shots from a high power rifle have punctured the spacecraft and tanks and lead to the series of events seen today?
I think the issue of the likelihood that this could actually happen would make a good parallel question, as I am sure there are several precautions in place.

edit 2: Tweet from ElonMusk - there are continuing developments and discussions, don't take this tweet out of context:

Particularly trying to understand the quieter bang sound a few seconds before the fireball goes off. May come from rocket or something else.

edit 3: and there's this stuff.
edit 4: Snippet from the Washington Post - please read the full article there.

...As part of the investigation, SpaceX officials had come across something suspicious they wanted to check out, according to three industry officials with knowledge of the episode. SpaceX had still images from video that appeared to show an odd shadow, then a white spot on the roof of a nearby building leased by ULA, a joint venture between Lockheed Martin and Boeing.
The SpaceX representative explained to the ULA officials on site that it was trying to run down all possible leads in what was a cordial, not accusatory, encounter, according to the industry sources, who spoke on the condition of anonymity because of the ongoing investigation.
The building, which had been used to refurbish rocket motors known as the SMARF, is just more than a mile away from the launchpad and has a clear line of sight to it...

They quote Musk as saying:

“We’ve eliminated all of the obvious possibilities for what occurred there,” he said. “So what remains are the less probable answers.”

Which almost sounds like the engineer's version of "...when you have eliminated the impossible, whatever remains, however improbable, must be the truth...".

A:

You'd have to get very lucky. Bullets would punch small holes in the RP-1 and LOX tanks and you'd have small streams of both flowing out. 
If the bullet were to hit one of the high-pressure tanks, however (like the helium tank inside one of the first stage propellant tanks), the tank could rupture and release its entire contents at once. 
If there was a rifle shot, I expect it would have been clearly audible in the video of the explosion. Apparently there was a sound just before the explosion (which I didn't hear on a casual review of the video), but it's not been definitively identified. 

A:

Max effective range of a .50 caliber rifle is on the order of 2000 yards (~1.8 km).  I'm trying to find a reliable number for the minimum safe distance from the F9 in the event of a kaboom, but I'm pretty sure that's well short of it (I want to say it's on the order of 3 km, but again, trying to find a source).  The shooter would risk severe injury or death.  

