The weapon in question in action:
https://www.youtube.com/watch?v=QCw_aAS7vWI&t=2m14sOn first look, it's a pair of forearm guards with embedded shotguns and custom shot payloads.
Pretty much anime physics involved, though, especially the 'selective' recoil.
The show is pretty much about superpowered people. And yes, all of the weapons in the series are hybrid melee/firearms. When asked if they would ever feature melee weapons that weren't also guns, the creator responded with, "Why would you ever want that?"
-k