New prop being used in Season 5
The new contingent of Marines are armed with a new white-coloured rifle with a visible air tank on it which some have assumed (edit - quite sensibly since they do mention using "rubber bullets") is intended to be a less-lethal weapon to pacify Happy Valley. It's not. It's the Hatsan 'Blitz' full auto Pre-Charged Pneumatic (PCP) air rifle - https://www.airgunseurope.com/hatsan-blitz-full-auto-pcp.html
With the iron sight setup removed and replaced with this optical sight - https://rmacleod.co.uk/vortex-razor-amg-uh-1-gen-ii-holographic-sight-002387/
I can't take screencaps using Apple TV unfortunately but you can pull up 42:00 and get plenty of good looks at these things.
Edit - It's still possible that in the show these are supposed to be for less-lethal use, or they are dual-purpose, or maybe the "rubber bullets" are just to prevent penetration of spacecraft hulls etc. We will hopefully find out.
Here's the dialogue:
"Teams three and four will insert here an assault through the substation, clearing any opposition using nonlethal means. I say again, flash bangs and rubbers only. We do not want to risk any ruptures in there."
In real life it's lethal at close range - up to 72 joules in its 7.62/30 caliber form - in theory enough to penetrate a skull at close range. And it would put holes in important things like spacesuits. In-universe it's quite possibly meant to be more powerful - modern larger caliber air rifles can generate basically modern handgun ballistics, albeit they are even larger in caliber than this (.357-.50 cal) and would not be capable of full auto which I'm guessing is going to come into play when they actually shoot these things. Interesting choice given the potential limitations of conventional firearms in a vacuum (not that they are likely to get used in vacuum in the show but may have influenced the choice to move away from the classic M16A2-based white rifles from Season 2 (or rather some modernisation of same). Alternatively the concern may be not putting holes in spacecraft.