Mythbusters" (which I didn't watch until Sunday night having DVR'd it), they fired a gun in a vacuum. This was to see if a gun could fire in space. (I didn't blog about this until today because I had to think about it for a bit because I was sure something was wrong.)
Now, I'm keenly interested in this subject because in my science fiction novel Rock Killer there are many guns fired in space, specifically on the Moon, in the first chapter. My research at the time of writing the novel led me to believe that guns would fire in space. Primers and propellant are self-oxidizing (they don't need oxygen in the air to burn) so the lack of oxygen wasn't an issue. The one thing I couldn't find a definite answer to was vacuum welding. Would the tight-fitting metal parts in a gun and/or cartridge vacuum weld in vacuum? Don't know.
I was very curious to find out how Mythbusters was going to test this. Because in space the vacuum is near absolute (interstellar medium has about one hydrogen atom per cubic centimeter) and that's very difficult and expensive to replicate that on Earth.
Way back when when Mythbusters set out to disprove the conspiracy theory that there was no Moon landing, NASA let them use one of their vacuum chambers that probably would come pretty close to space conditions. But I sincerely doubted NASA was going to let them fire a gun in that gazillion-dollar vacuum chamber. And I was correct.
The Mythbusters built a vacuum chamber out of bullet-resistant plastic (which meant long seams to leak air through). And yes, they made a partial vacuum in that chamber. The problem is there is an asymptotic relationship. As you approach a perfect vacuum, the energy needed to extract those last few molecules reaches infinity. So the lower vacuum you want, the more energy it will take to create and maintain it. They didn't show the vacuum pump used but I doubt it was very big (it wasn't looming in the background).
According to Wikipedia, outer space has a gas pressure of 0.0001 Pascal (Pa) or one ten thousandth of a Pascal to less than 0.000000000000003 Pa. That's three quadrillionths of a Pascal. Since atmospheric pressure is about 101,300 Pa (or 14.7 pounds per square inch) the air around you is about one billion times higher than the highest gas pressure in space. (The actually atmospheric pressure you experience depends on air temperature, your altitude, and the weather you are experiencing.)
And, according to Wikipedia, it is possible to get about as low as one millionth of a Pascal of vacuum on Earth which is below the upper range for space. But that's using very specialized and expensive equipment.
The Mythbusters had a vacuum gauge on their vacuum chamber and when they fired the gun it read -90 kPa (or -90,000 Pa). Atmospheric pressure is 101,300 Pa as stated above. So they got the gas pressure in their chamber down to 11,300 Pa (101,300 - 90,000) which is a very, very poor vacuum. In fact, 11.15% of atmospheric pressure remained and the pressure in the chamber was 1.64 pounds per square inch. Hardly space-like conditions.
The gun fired, but this was nothing like the conditions in space. Their vacuum had 113 million times more pressure than the highest pressure in space (0.0001 Pa).
Even if you use the Karman line (the point above the Earth, 100 kilometers above sea level where traditionally space is said to start) as your definition of "space" the gas pressure there is 0.032 Pa and the Mythbusters "space" had 353,125 times more pressure than that.
So it's still an open question if a gun will fire in space.