Being a physics moron, I have no idea, but when a friend brought this up last night I told him “I’ll find out for you by tomorrow.”
SO, what effect would the reduced atmosphere and lower gravitational pull have on the speed of a bullet fired from your standard handgun?
The bullet would travel faster and straighter due to lack of wind resistance. It would also travel farther due to the lower gravity.
And it would fire, too. The powder in the casing contains everything it needs to ignite, even in a vacuum.
Any idea how much faster?
OK, here’s my WAG.
The initial speed wouldn’t be all that much faster (the explosion by far outweighs the friction of the air on earth, thus resembling a theoretical vaccuum for a very short time). The bullet would, however, be slowed down less quickly. So, the end speed (before the gravitational pull forces the bullet to the moons surface) WILL be significantly higher than on earth. But at the moment of firing the gun, the speed difference is negligable.
Damn…Colfire beat me to it.
What Coldfire said…
Also, the bullet would travel further since the lower gravity would allow the bullet to remain in flight longer before hitting the moon.
Would it differ for something like a missle? (Sorry, I guess I should have put this in the OP, since it was part of the discussion last night.)
Actually, it would. A missile uses a propulsion system that NEEDS the friction of the Earth’s air. So my WAG would be that a missile would NOT go as far and fast on the moon as it would down here, assuming a horizontal take off just like the bullet.
I’m not so sure about that. Most guns fire bullets at supersonic speeds, right? It takes a lot of energy to push something past the sound barrier. I’d guess that a bullet would be measurably faster on the moon. I have no idea how much though, sorry.
Supersonic speeds? I don’t think so, really. But then, I know very little about guns. My WAG would be that your average handgun would fire at about 500, 600 km/h here on Earth.
Also, the sound barrier is probably higher on the moon, so the bullet might (hypothetically) have to break the sound barrier here, but still remain subsonic when fired on the moon.
My head hurts 
This depends on the missile. This is definitely true for cruise missiles which use jet engines. Ballistic missiles and, I think, smaller missiles (e.g. air-to-air missiles) use solid fuel rockets, which work perfectly fine in vacuum. The nozzle shape is optimized for atmospheric use, so it would be less efficient, but you have no air to push through, so whether it actually goes faster or slower is hard to say. I’d guess most missiles would fly faster.
The only 2 common guns that are SUB-sonic are the .45 A.C.P. and the lowly .22 (which, by the way, is why they are such a popular choice for hitmen. You can silence any gun but not its bullit if it is going faster than the speed of sound. It creates its own little sonic boom)
I meant, bullet, of course.
I stand corrected!
Hey, I like to WAG
The majority of bullets on Earth are supersonic. Only a few (some .45’s, for example) are subsonic. My rifle fires rounds at a muzzle velocity of about 3000 feet per second (914 m/s), whereas the speed of sound is about 1088 ft/s (331 m/s). Roughly mach 3.
And just a little nitpick: There is no speed of sound on the moon, since there’s no atmosphere. 
When the bullet hits the ground (or the target) it will be going exactly the same speed as when it left the barrel of the gun, plus a tiny bit more, due to acceleration from gravity.
I’d say it’d travel farther: no air to resist it and lower gravity so it won’t fall to ground as fast. I do know that on Apollo 15 ( I think) one of the astronauts snuck a driver and three golf balls up there for a whack. First one went and estimated 1/6 of a mile.
Wouldn’t the greater pressure difference between the propelling gasses and the near vacuum of the lunar atmosphere make it go faster out the barrel?
You could get a crude estimate for the bullet’s increased velocity when exiting the muzzle by assuming bullet on Earth has mass equal to the bullet plus the mass of the air ahead of the bullet in the muzzle, and equating momentum at the exit.
M[sub]bullet[/sub] * V[sub]Moon[/sub] = (M[sub]bullet[/sub] + M[sub]air[/sub]) * V[sub]Earth[/sub]
or V[sub]Moon[/sub] = V[sub]Earth[/sub] * (1 + M[sub]air[/sub] / M[sub]bullet[/sub])
This is probably an over-estimate, but it at least tells you the order of magnitude of difference you can expect. I don’t know bullet masses, so i’ll leave it to someone else to fill in the numbers.
Would the cartrige even be able to fire with no oxygen? I’m looking at the controlled explosion as a physical reaction versus chemical.
Yes. We’ve been over this a few dozen times on the “which bullet will hit the ground first” threads. The propellant used in bullets contains an oxidant. If it had to depend on atmospheric oxygen to burn, it would be pretty useless. There’s no way enough air could get in there to cause it to burn fast enough to propel the bullet. Don’t forget that the cartridge is sealed when it’s fired. No air can get in there at all.
In the nuts and bolts physics of bullets and guns there are some other things that change the speed of the bullet as it is fired. One of these is the compression of air in the barrel in front of the bullet as the compressed gasses behind the bullet accelerate it. For short barreled weapons like pistols, it is a minor consideration. For long guns, such as rifles, it is a much more important factor.
Accelerating the bullet along an evacuated barrel would be much more efficient. The friction while passing through the air after leaving the barrel would matter more for range, but initial velocity is much more strongly affected by the differential pressure between the firing chamber, and the barrel. With no atmosphere at all that differential is a lot greater.
Tris