See the wires? The smaller tubes on the bottom are batteries. Japan has a lot of fireworks. He probibly use electric igniters for fireworks or maybe just simple broken light bulbs. With the gunpowder from a disassembled firework. Flip the switch and boom. With a hot load(too much powder for the barrel strength) you couldn't fire it more than a few times or it would blow up in your face. Plenty effective for a suicide mission.

What surprises me is that no one else was hit. I find that very strange.