🇯🇵🔫❗️— A higher quality photo. The hardware store pipe and end caps are clearly visible. It appears that the firing method was electric, judging by the wires and connectors on the grip. This is a fairly advance design when compared to the standard "four winds" style slamfire shotgun. It makes sense within the context of Japan, where even acquiring live shotgun shells is of great difficulty. It was easier for the assailant to ignite the black powder using electric means rather than percussive.

Apparently, the guy had an arsenal.