My limited understanding of this is that you can buy the critical gun parts as spare parts from US websites. Much less likelihood of getting caught if it's just a 6 cm piece of machined metal turning up in the mail. Then you can 3D print everything else and assemble.
Homemade shotgun. Guns are pretty hard to come by in Japan, so homemade is probably easier than procuring one, esp if you have da skillz.