Sci-fi dorks don't want you to know this, but pump action shotguns will still be used because they don't care about different alien atmospheres, and are less likely to puncture a ships hull.

pump actions are reliable but don't care any less about atmosphere than automatic actions, gas-blowback or recoil operated firearms produce their own gasses/forces (from the bullet propellant) and contain their own oxidizers (no need for atmosphere)

basically just get a power armor suit with a full-auto multipurpose flechette/microgrenade launcher mounted to one of the arms or your shoulder.

Star Wars battlefronts 2 (2005) engineer class predicted this

pump action shotguns will still be used because they sound really cool

You watched that Forgotten Weapons video didn’t you.

Shot guns and swords

fuck yea bro

I wonder what the Borg would do against buckshot? You'd think the Federation would stock some kinetic small and large arms for the odd species that shrugs off phaser blasts.

IDK, even pump shotguns would be subject to issues with vaccuum environments , but I guess differences in atmospheric pressure in alien environments might be an issue if it changes how fast the gas disperses.