In the sci-fi drama For All Mankind, there is a scene in which an American soldier in space uses an M-16 rifle. Is this scene scientifically valid? Let's look at them one by one.
Once in outer space, including the lunar surface, triggering the M-16 rifle will trigger the ammunition. This is because the ammunition contains its own propellant and oxidant, and the combustion of the propellant does not depend on external oxygen. What's more, properly crafted military ammunition has a seal that protects the interior of the ammunition from the outside environment. And since space has no gravity to draw the fired bullets to the ground, and there is no atmosphere, the wind does not blow, so the straightness of the trajectory will be much better. Bullets fired from outer space without gravity and the atmosphere will fly virtually forever unless they encounter other obstacles, especially considering that bullets fired from Earth lose most of their kinetic energy during flight as air resistance. The moon's surface has 1/6 of the Earth's gravity, so the fired bullets won't be able to fly forever, but the distance (maximum range) it will fly until it lands on the ground will be much longer than that of Earth.
However, it is difficult for the operation after the bullet is fired, that is, the bolt retreat, the ejection of the cartridge case, the bolt advancement, and the reloading of the shotgun normally. The M-16 rifle is a weapon designed on the premise of being used in a global environment. The environment on Earth has an atmosphere with an acceleration of gravity of 1G and an atmospheric pressure of 1 atm above sea level. The temperature range recorded so far is about minus 90 degrees Celsius and about 57 degrees Celsius.
On the other hand, the environment of the universe is markedly different from that of the Earth. There is basically no gravity and no atmosphere in outer space. In the case of the moon described in the drama, gravity is 1/6 of the earth. There is no atmosphere. The moon's temperature is as low as -173 degrees Celsius and 126 degrees Celsius.
Earth's environment affects all parts of the M-16 rifle. Especially the gravity. Gravity affects everything from the way ammunition is contained in the magazine to the way the double-seat spring advances the bolt. The operation of the M-16 rifle is based on the existence of this gravity, and the sequence and timing are very precise. As a result, the moment gravity disappears, all this operating procedure is messed up. For example, the bolt of an M-16 rifle retreats by the force of the explosive gas from the gunpowder in the ammunition, because in zero or low gravity conditions, this explosive gas does not work the same as on Earth. For this reason, M-16 rifles for use in space must be completely redesigned for zero-gravity environments to ensure normal operation. This means that if you take what you used on Earth into space as it is, the bolt retreat may not be done properly.
After that, the moon's temperature becomes a problem. If the M-16 rifle is placed on the lunar moon, the rifle will be heated to 126 degrees Celsius. When shooting in that state, the firearm will overheat much faster than when shooting from Earth. If the firearm overheats, the chamber temperature rises, which can trigger a spontaneous triggering of the firearm itself even if the trigger was not pulled. If it gets worse in that state, parts that are subject to a lot of stress such as barrels and gas shafts may melt and deform. Even on Earth, there have been cases where rifles were fired continuously without a break (except for magazine exchanges of course), and then fired more than 400 automatically and eventually, the barrel melted and exploded.
What if it was left in the shade? It is exposed to minus 173 degrees Celsius. Some firearm lubricants harden at this temperature and become solid. In order to operate the M-16 rifle on the moon, it must have a lubricant that remains liquid at -173 degrees Celsius.
There is another problem with the surface of the moon. It is moon dust. Moondust composed of basalt is fine dust with a particle size of 10 to 50 micrometers or less. Also, like dust generated only by mechanical weathering, it is very coarse, which is harmful to both the human body and equipment. It rises up to an altitude of 100 km under the influence of radiation hitting the moon's surface. For that reason, in order to operate the M-16 rifle on the lunar surface, the rifle must be tightly sealed to prevent lunar dust from entering. Otherwise, the dust entered will interfere with the firearm's operation and may gnaw the surface of the parts.
Even if all these problems are solved, there is a problem that cannot be helped. It is the treatment of recoil. The Moon's gravity is weaker than the Earth's, making it difficult to control recoil. It would be even more difficult if there was no gravity at all. When shooting in outer space, the shooter's body is continuously pushed in the opposite direction of the bullet's firing direction. Unless you apply a separate brake.