Something about the whole "shooting guns in space" thing has been bothering me all week, though I couldn't articulate why exactly until it literally just came to me while thinking about something else (entirely unrelated). and that is put simply: thermal build-up. Yeah, they mentioned that the reason they painted them white is to reflect as much IR from the sun as possible, but what about the heat generated simply by firing a round? One assumes the heat dissipation on an M-16 was designed with air cooling in mind, not hard vacuum, so I doubt the existing heatsink wouldn't be able to keep up with the thermal load for very long. That means that at best the guns will overheat and jam after just a few rounds (honestly I'd be shocked if they even get through a whole magazine), at worst, not only will it jam, but the rounds will start to cook off in the chamber and explode, likely killing the operator and anyone unlucky enough to be standing next to them. They'd need to rig up a much more robust heatsink, maybe even a waterloop with a large sink installed on the backpack for maximum dispersion and less bulk on the weapon itself.
As if their shock at the cosmonauts burning wasn't evidence enough (I mean come on guys; flamible material + pure oxygen mix + extreme heat = space barbeque), it's clear nobody at the DoD really put any serious thought into this.
I think they'd have been better off rigging up some kind of flechette gun, preferably one that doesn't need explosives to work; less mass, less energy, less of a danger of ricochets, less chance it'll breach a hab's shell.
With the low gravity and zero air resistance, the range on even a small normal firearm would be WAY beyond an operator's ability to aim reliably, so most of that explosive energy is not only dangerous, it's overkill.
It may be counter-intuitive, but for small arms combat on the moon, something a lot closer to a crossbow might be the smarter choice.