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Does the gas come from the combustion of the powder in the bullet cartridge?
It wouldn't surprise me if that's one thing they're using X-37 for, is a place to test fire rifles safety in space.
Technically, the gas operation of any gas operated weapon will work perfectly fine in the vacuum of space.
Technically.
However, as you rightly ask, does the propellant create hot gases, or does it heat already existing gas present. My thinking is that it creates hot gases, similar to an SRB. In that case, a 5.56x45 NATO round will work just fine in vacuum.
I can't watch the video, so I'll assume the slow movement isn't a combination of filming effects and the whole 'everything' moves slow on the moon' trope and consider why the bolt would move slower.
In the real world, the gas imparted on the system will move the bolt carrier backwards at speed until it compresses the recoil return spring and buffer plate enough so it gets pushed forwards again to complete the cycle. On the Moon, because the bolt carrier will actually be 1/6th its mass on Earth, will only need 1/6th the force required to cycle the weapon. I think NASA will be using much lower grain propellant for this reason as it doesn't want the bolt basically hammering the weapon apart as it cycles.
Also, the human firing it will be 1/6th his mass. But the recoil impulse fired will still be the same and will effect him 6 times more than usual, another reason to 'dial down' the round.
Sustained burst fire could potentially lift him off his feet entirely.
That won't make a huge problem for accuracy and lethality, as the low gravity and zero atmosphere means bullet drop and muzzle velocity reduction will be almost non-existent and effective range almost infinite.
Theoretically, 5.56mm rounds fired from the surface will become at least sub-orbital, if not mini-death satellites, constantly circling the moon at 900+m/s until their orbit decays or they strike a solid object. And it doesn't need to even cause huge amounts of damage to a man to become effective, like on Earth. All the bullet needs to do is cause sufficient damage to an enemy combatants space suit, compromise its integrity and the occupant is dead regardless. Hell, at those velocities, you'd be able to engage and over-penetrate landers, habitats, craft in low orbit and the like thanks to their extremely thin aluminium construction.
Much lower muzzle velocities are required for space combat, and I think another good, safe reason to 'dial back' the bullet.
I mean, there are a million other problems NASA would need to solve before they use firearms in space. Ammunition cook offs and extreme weapon heating / cooling due to sunlight being big ones.