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.
a pump shotgun no, but potentially some gas operated systems? i'm not really sure how gravity/vacuum would affect any recoil operating system like that.