They look to be shotguns, which would fit with the CQB fluff-especially for boarding actions, where a round of buckshot can ricochet around a corner, off the walls, etc., and do more damage than a single solid round. Just my $.02, though.
Yep. They deviates a bit from the standard 40k storm trooper model. But there are a lot of Guard 'outfits.' The Stroikan 5th are CQB specialists and often aid in naval breaching actions. At least, that's my excuse for the re-breathers and fully enclosed helmet