Having messed with firearms most of my life, this looks like a homemade shotgun made from pipe, and since guns (and therefore ammo) is not readily available in Japan, the shooter probably just crammed available particles (gravel, nuts and bolts, broken glass, or whatever) in front of the powder charge. Also, since brass and primers are most likely unavailable as well in Japan, he probably used powder extracted from fireworks and the like to make enough to launch whatever he was using for ammo. He MIGHT have used a single slug, maybe a plug of metal that would fit in the pipes, or some combination of materials.

I'm sure the Japanese police will be studying the homemade firearm for some time to come, and we may never know exactly how he did it. But the technology is quite old, and anyone with more brain cells than a Democrat could put together a lethal gun, although it would be primitive by modern standards.