From the looks of the Banzai Blaster 2000, it seems to be just common hardware store pieces with some electronics from either airsoft or RC cars, both of which are big in Japan. Under the barrels you can see what looks to be a Ni-Mh battery from either one of the hobbies. Black powder or propellants like it aren't too hard to make from unregulated components.
I wonder how this'll be spun into an argument for gun control, seeing as there's no 3D printed parts and 3D printed "ghost guns" are the scariest, most marketable kind. They certainly can't ban airsoft, as that's the only reason 90% of Japan's NEETs even bother leaving the house.
Curious as to the shooter's motives. It's one thing to leave a 3D printed single-shot "disposable" pistol designed by someone many times smarter than you printing overnight, while jacking off to cartoons. It's an entirely different beast to cobble together a hardware store zip gun and have the confidence in your craftmanship to actually try to use it. Seems to me this guy didn't do this out of the blue, and put a modicum of thought into it.