In a climactic scene in the new sci-fi actioner "Total Recall," our heroes Hauser (Colin Farrell) and Melina (Jessica Biel) find themselves caught in a zero-gravity shootout near the center of the Earth. Would guns really work in zero gravity?
Yes. Unlike most ballpoint pens, for example, which are famously ill-equipped for the weightlessness of outer space, gravity has nothing to do with the mechanical functioning of a gun. Instead, both semi-automatic and automatic guns rely on springs (not gravity) to bring the next bullet into the chamber, before a small explosion within the gun fires the bullet.
However, as in the film, a zero-g gunman should exercise some caution, as firing a bullet would also send him or her flying in the opposite direction. When you shoot a gun under normal conditions on Earth, the friction between your body and the ground keeps you in place. With every shot you experience recoil (your arms and shoulders are forced back by the force of the bullet's projection), but with an experienced shooter it's only about an inch to a foot, and you should have no problem staying on your feet. In zero gravity, on the other hand, even the smallest recoil would send you backwards. In most cases this would be very manageable, however, bouncing you back at a speed of less than one meter per second, so you wouldn't have to worry about seriously injuring yourself. Even if you fired a .44 Magnum, for example, and weighed only 100 pounds, the recoil velocity would be under 0.5 mph, which is still less than walking speed. If you wanted to really propel yourself using a gun, you'd want to use a really big one.
What about the bullet? Would it be more quick and deadly in space? Not really. The speed of the bullet would remain roughly the same in zero gravity as it is on Earth, so shooting in space is not going to make your gun any more or less lethal to your space enemy. The only way you'd see a noticeable difference is if you were firing the bullet over a long distance. On Earth, a combination of air resistance and gravity slows the bullet and gradually pulls it down towards the Earth. This is why when you shoot your weapon, you have to aim slightly higher than your target. This force is so small, however, that over short distances there would essentially be no difference in the shot's trajectory.
Thanks to L. Nelson of Bishop's University and L.P. Brezny of Metro Gun Systems.