There’s a comic strip called Non Sequitur. Not one of my favorites, but occasionally there’s a pretty good one. Here’s an example. We have a kid speculating about firing a gun on the moon. He asks:
If you were on the moon, which is a vacuum, and tried to shoot a gun, would it fire? After all combustion needs an atmosphere with oxygen… or does the casing of the bullet create its own atmosphere? If so, the bullet would travel farther in the reduced gravity, but would it travel faster than it does on the earth?
Let’s take ’em one at a time.
First, the bullet would fire. Explosives including gunpowder don’t need atmospheric oxygen to fire. They react far too quickly, and atmospheric oxygen can’t be pulled in from the surrounding air fast enough. Instead, explosives contain their own oxidizers and will work in a vacuum just fine.
Willthe bullet travel farther? Sure it will. A bullet stops moving once it hits the ground, and if it’s falling more slowly in the low gravity it will go farther, all other things being equal.
Will the bullet travel faster? No, not appreciably. It won’t lose any speed until it hits the ground and stops, but the lack of air won’t make it come out of the barrel any faster. I suppose there won’t be any air resistance in the barrel itself, but that shouldn’t do anything significant.
Now the comic itself shows the bullet being fired, coming all the way around the moon, and hitting a rock near the shooter. Is that doable? Well, an object in circular motion has a relationship between acceleration, velocity, and the radius of the circle given by:
On the moon, the acceleration due to gravity is about a = 1.622 m/s2.
So we can use that fact and find the velocity necessary for a bullet to be in orbit just above the moon’s surface:
That’s about 5,500 feet per second. A standard 9mm bullet travels at about 1000 feet per second. Not enough. A .30-06 rifle bullet reaches around 2,900 feet per second. Still not there. The fastest commercially available cartridge I know of is the .17 Remington, which screams along at 4,400 or so feet per second. That’s close but not enough either, and it’s already an uncommon specialty round that’s about at the limit of the abuse a barrel can realistically take. So I’d say that part of the comic isn’t possible.
For the best, I think. The last thing the moon needs is a lot of bullets orbiting a few feet above the surface. That would probably be a bit hazardous for future exploration.