:dubious: A vacuum is neither hot nor cold. A vacuum, by defintion, has no temperature beyond that provided by any radiation passing through. IOW a vacuum in the same orbit a the Earth will be incredibly “warm” in full sunlight and incredibly “cold” in the shade.
Moreover a vacuum can’t dampen out anything. A vacuum is an absence of matter. With no matter to absorb energy or combine with reactants there is no dampening process possible.
The Russians have included a firearm in their Soyuz survival kit for decades. It’s a useful item if you land in some remote region of Siberia.
As did the crew of Voskhod 2 
Using a gun on the moon was included in a lateral thinking test Dad got from work. The idea was that you’d point in the opposite direction you wanted to move in and shoot to propel yourself across the surface of the moon without having to walk this saving energy.
While I can see the eloquence in this solution, I have to wonder at its practicality. To keep from falling over, the astronaut would have to jump then fire and worry about landing. Even then, there would be the question of how far a shot would propel the astronaut. I would guess the mass differential between a fully loaded moonwalker and a .45 slug is pretty great.
Maybe if they were to carry sawed off shotguns loaded with slugs.
Great, now I have images of Gangsta Astronauts in my head. There goes production for the rest of the day.
Why bring along firearms?
Bend over, pick up a rock, jump, throw rock in the opposite direction of where you want to go, land, repeat.
Probably easier to just bounce along.
Ah, the raft is a nice touch 
A 230 grain .45 ACP bullet moving at 900 feet per second would have a momentum of about 4 newtons, or 0.9 lbf. (That’s about 29.5 poundals, for any psychotic who wants to use that system.) This isn’t enough to move even a small person more than a fraction of an inch. Even a shotgun with slugs wouldn’t shift a person more than a couple of inches, even in the Moon’s gentle field.
Stranger
If not a gun, do explosives count?
Apollo active seismic experiments
Too bad the rover cameras were deactivated before the mortars were set off. I would have liked to have seen those fireworks.
Firing a bullet from a handgun in space is a pretty extreme step. You are going to tumble, for sure, and the bullet is going to be a navigational hazard as well. It will have a new orbit, that intersects with your orbit once each orbital cycle. So, from any useful orbit, it becomes a big problem for you. If you hit your target, the orbital mechanics of hazards change, but mostly just the number of projectiles from your shattered target.
There is way too much trash in orbit already. Target practice doesn’t seem warranted.
Tris
Wouldn’t a bullet fired from a gun already in Earth orbit have enough velocity to leave Earth orbit?
Check your units, there. Newtons and lbf are both units of force, not momentum. The unit for momentum doesn’t have any standard name, beyond “kg m/s” (although many intro physics teachers have nonstandardly attached their own names).
This question might completely display an ignorance of the scope of the moon, but could a bullet fired from the surface of the moon enter moon orbit? Say from the highest powered rifle we have available?
Urk! I neglected to include the seconds on those momentum numbers. That should be 4N·s and 0.9 lbf·s respectively. It’s a common convention in rockets to speak of impulse in units of force, even though the proper units are force·time, and of specific impulse (“weight specific impulse”) in seconds rather than impulse/mass (lbf·s/slug or N·s/kg).
The escape velocity at any altitute is √2 of the circular orbital velocity at that altitude. For an escape velocity for a bullet moving 900 ft/s (0.274 km/s) the orbital radius would have to be r = 2μ/v[sub]e[/sub][sup]2[/sup] = 21.2 million kilometers. This is well beyond the Moon’s orbit and indeed outside of Earth’s nominal sphere of influence (SOI), so any spacecraft at this point would by default be in orbit of the Sun rather than Earth. (Obviously, there’s a closer boundary at which a spacecraft would transition from Earth-oribtal to Sol-orbital, but that depends upon the position and relative velocity of the spacecraft and bullet.) At Low Earth Orbit the projectile would just become another piece of hazardous orbital debris.
Ditto for the Moon. The speeds and momentum at orbital velocities are truly beyond anything we experience in everyday life.
Stranger
Thanks for the answer, that’s surprising (to me at least). I would have thought the bullet would just head out on it’s merry way.
Any idea what the (average diameter, I’m guessing it would start out eliptical)orbit would be for say a 180 grain bullet fired at 2500 fps from LEO? Could you hit the Moon with such a shot (or at least intersect its orbit) fired from LEO?
Actually, I screwed up by an additional factor of two; the correct radius should be about 10.5 million km, still well outside the Earth’s SOI. I should note, however, that I’m assuming firing from a stationary (non-orbiting) platform there. If you fire from a platform in orbit in the tangential direction (or indeed in any direction that doesn’t have a component opposing motion or below the horizon) then the escape velocity of your bullet relative to the platform reduces to v[sub]bullet[/sub]=(√2-1)·v[sub]o[/sub] and the radius is (√2-1)[sup]2[/sup]·μ/v[sub]bullet[/sub][sup]2[/sup], which gives a mere 911000 kilometers, which is still about two and a half times the orbital radius of the Moon.
At LEO (200-2000km altitude) escape speed varies from 11.0 km/s to 9.8km/s, and orbital velocities are 7.79 and 6.90 respectively, so in order to escape you’d have to make up the difference. (If you can come up with a man-fireable gun that can shoot projectiles at ~3km/s I can find you a multi-billion dollar defense contract.) At 2500 fps (0.762 km/s) you’d have to have an orbital radius of 118000 km, almost a third of the way to the Moon, and way higher than geostationary orbit (GEO, 42200 km orbital radius). The mass of the projectile is immaterial, assuming that it is significantly smaller than the bodies exerting gravitational influence upon it.
We’re used to seeing images of the Space Shuttle in orbit, slowly nudging its way toward a satellite, and thinking, “Boy, those things don’t move very fast.” But they’re just not moving fast with respect to one another; relative to an observer on the ground, they’re moving about an order of magnitude faster than a bullet. This is why talking blythly about the ease of “hit-to-kill” intercepts of ballistic missiles or launching the spacecraft too the Moon enjoins such scathing laughter among engineers and scientists. It’s not impossible but it’s very, very difficult, requiring a lot of energy and very precise guidance and control. When I look at the numbers, I’m amazed that we actually managed to seen Apollo capsules to the Moon, repeatedly, without getting lost or putting the astronauts in a deadly slow overshoot orbit.
Stranger
Thanks again, this is fascinating reading.
I suppose the lack of atmosphere in the gun barrel would have a insignificant effect on muzzle velocity?
It would probably be something measurable, but I wouldn’t expect much. Air compresses readily, and the difference in pressure between the ambient atmosphere in front of the bullet and the propellant gases behind it is huge. I’d guess the difference to be no more than a few tens of fps, if that; less than the typical variation from round to round.
Stranger
So what would happen if you were on the ISS and fired a round towards the Earth? Would it drop to a lower orbit, or would it reenter the atmosphere and burn up?
Bad idea. The ISS has an average orbital speed of about 7.71 km/s–considerably more than any bullet you might fire from it. Firing laterally is just going to increase the velocity (summing the velocity vector of the ISS with that of the bullet wrt the station), plus it’ll come back around to intersect the orbit of the ISS, resulting in a small but not infinitesimal probability that it’ll impact the station at some point. If you want to put the bullet on Earth, the best thing to do is to fire back into the path of the station, thus cancelling some of the velocity and driving it into an elliptical orbit that goes down toward the planet. While it’ll still nominally intersect the station’s path, in reality it’ll tend to drift down lower as it sees some drag from the thermosphere, eventually slowing down into reentry and disintegration. Remember, with orbital mechanics, you speed up to slow down (go to higher orbit), and you slow down to speed up (go to lower orbit). In any case, you couldn’t cancel enough velocity to just shoot a bullet down to Earth; this would take 7-9 km/s of delta-v, dependant on where you want it to land, vastly more than you’ll get out of any kind of small arm.
Stranger