What Would Happen If You Shot a Gun In Space?
Fires can't burn in the oxygen-free vacuum of space, but guns can shoot. Modern ammunition contains its own oxidizer, a chemical that will trigger the explosion of gunpowder, and thus the firing of a bullet, wherever you are in the universe. No atmospheric oxygen required. The only difference between pulling the trigger on Earth and in space is the shape of the resulting smoke trail. In space, "it would be an expanding sphere of smoke from the tip of the barrel,"
If you do the latter, Newton's third law dictates that the force exerted on the bullet will impart an equal and opposite force on the gun, and, because you're holding the gun, you. With very few intergalactic atoms against which to brace yourself, you'll start moving backward (not that you’d have any way of knowing). If the bullet leaves the gun barrel at 1,000 meters per second, you — because you're much more massive than it is — will head the other way at only a few centimeters.
Shooting someone in the back is a cowardly act. In space, "theoretically you could shoot yourself in the back,"
You could do it, for example, while in orbit around a planet. Because objects orbiting planets are actually in a constant state of free fall, you have to get the setup just right. You'd have to shoot horizontally at just the right altitude for the bullet to circle the planet and fall back to where it started (you). And you'd also have to consider how much you'll get kicked backwards (and consequently, how much your altitude will change) when you fire.