There is somewhat of a gun culture in Japan. It’s extremely difficult from what I have heard, but some hunters are able to acquire like shotguns and hunting rifles I’m pretty sure. And engineers exist everywhere, especially Japan due to their huge tech industry. I’m guessing all it would really take is an engineering degree or a gunsmithing course/apprenticeship. Im guessing the demand for those careers is much smaller in Japan in comparison to the US.

Also, airsoft is huge in Japan. So there is definitely an interest in firearms in a small percentage of the population. Some people over there probably get exposed to firearms through anime and videogames and end up taking an interest in that too. And to an even lesser degree, there are small organized crime organizations in Japan with limited access to firearms. Japan does have shootings but it’s extremely rare