Veraly, that’s a good idea.
So the bullet will strike point-first?
Yup. It has no reason to tumble unless something is wrong with the gun or ammunition.
Perhaps at very long ranges, microscopic flaws in the barrel could cause the bullet to tumble enough to hit flat, but if it’s a decent weapon it should fly point first for a long time.
Actually, it is about the atmosphere. The purpose of spin stabilization is to keep the projectile in an aerodynamically stable orientation. If a bullet is tumbling in an atmosphere the path can be drastically altered over long distances. Absent air resistance it doesn’t matter how the slug is tumbling. There is nothing for the changing surface to react against.
Yeah, if you get dressed up nice you go places.
Why do you think recoil would be a problem with low gravity? F=ma still applies even in zero gravity. The only worry you’d have with recoil is if it wore a hole in your pressure suit.
Low gravity and lack of air would improve every kinetic weapon, given them seeming laser-like accuracy. Probably the best all-around weapon would be a short-barrel shotgun (possibly a shot handgun) with a wide choke. Since all it really has to do is penetrate a pressure suit, it could be low energy and maybe even built into the forearm of the suit.
For anti-equipment weapons, carbine versions of Earth’s most powerful sniper rifles would probably be very useful.
More accurate, yes, but hardly laser-like. Bullets don’t exit the barrel exactly parallel to it.
well it’s as near as dammit.
Depends on the gun. You aren’t going to get long-range accuracy with a short barrel, even on the moon.
Quercus already went over this, but the main problem is that most of the other forces (and torques) on you are proportional to your weight, while the force (and torque) from the recoil is going to be the same as it always was, so it would be easier for the torque from the recoil to overcome the other torques and start you rotating (and hence, falling). That said, I think that recoil is significantly overstated in popular culture, so it probably still wouldn’t be a huge problem.
As I said before, this may work initially, but soon they would realize that people are dying to suit tears, and would make the suit itself out of something much stronger, such as kevlar, and a low powered shotgun doesn’t penetrate very well.
Additionally, since depressurization would indeed be a large issue, they would probably take to mechanical counter pressure spacesuits. A hole or tear in those would be painful, of course, but no more so than a bullet wound already is, and not immediately life threatening unless the hole is in the helmet or lungs. Any hole would be naturally sealed by flesh pressing out of the hole, which would leave a painful bruise, but easily survivable. Additionally, the reduced weight on the moon would mean they could be covered head to toe in a very generous application of kevlar or other tough armor, further reducing the effectiveness of piercing weapons.
Even with a normal suit, there are certainly fluids that will congeal and harden when exposed to low pressure. Lots of people use this sort of thing in tires, which would make suit penetrations much more survivable.
If the system of rifle + human body were completely rigid & fixed, I might agree, but with the human body flexing in 3 dimensions, and the rifle able to recoil vertically, I don’t see this being a problem. The stance might need to be a trifle wider, if even that.
The problem of armor bulk would be the same on earth as it is the moon. It would still be as big and limit motion.
Anyway, the question isn’t whether there’s any weapon that is completely unstoppable on the moon, the question is what’s plausible enough to where you’d have to think to counter it. The answer is that firearms would work, they’d be accurate to longer distances, and in general, they’d be more effective at lower energies. It would take more effort and expense to counter small firearms on the moon. Concussive weapons would be completely ineffective.
I admit I just skimmed over the thread, so someone might have brought this up but it slipped under my radar, but…there actually was an Army design study for a lunar military base in 1959, Project Horizon.
It includes a relevant bit…“The base would be defended against Russian overland attack by man-fired weapons - unguided Davy Crockett rockets with low-yield nuclear warheads, and conventional claymore mines modified to puncture pressure suits.”
Needless to say, the whole concept was pretty theoretical, and slightly wildly impractical, and nothing serious came of it.
Hah! 
Yeah, on the Moon, a Davy Crockett might even have a longer range than the blast radius.