Blog post: swansont: In Space, No-One Can Hear You Go "Kerpow!"

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 per second.