A piece of pipe with a drilled end cap, a battery, a switch, a bit of nichrome igniter, some homemade black powder, some wadding, some 00 size ball bearings.
Voila, a shotgun.
The mind is the weapon, everything else is tools.
No need for home made black powder, there is a rather large fireworks market in Japan. Just cut open some mortar shells.
When I was a kid, we made .22’s out of car antennas, .410 & 12ga cannons out of plumbing pipe and a nail. It’s not rocket surgery.