These are identified in the Alien RPG as "EVA pistols" and in the official novelisation as "laser pistols".
Dallas, Kane and Lambert made their way down a narrow corridor. They now wore boots, jackets and gloves in addition to their insulated work pants. They carried laser pistols, miniature versions of the welder currently being used by Parker and Brett.
Alien: Official Novelisation
and
Alien RPG: Core Rulebook
These are evidently single shot laser weapons, primarily intended for self-defence and not very accurate, as you'd expect from a gun that's going to be used by the equivalent of space truckers.
The RXF-M5 was essentially a miniaturized, weaponized version of commercial laser welders that were in use in the early 2100s.
The official novelisation explains why they don't use them, for fear of causing the alien to bleed. The RPG also mentions that they're "armour piercing" which would also be an impediment to using them on a spaceship where explosive decompression would be a risk
‘How would we get it into the net?’ Dallas asked quietly. Brett considered. ‘Have to use something that wouldn’t make it bleed, of course. Knives and sharp probes of any kind are out. Same goes for pistols. I could make up a batch of long metal tubes with batteries in them. We’ve plenty of both somewhere back in stores. Only take a few hours.’