You would have to contact the Japanese version of the BATFE to find out.
Other than a toy gun or water gun, the next most powerful one a citizen can have without a permit is a "B.B. gun".
Well it was a 8.9 on the Richter scale. 5th most powerful in the world. MOST powerful in Japan's history.
Japan has the most powerful economy in Asia.
577 T Rex
star cannon
The magnum .55 is the most powerful, but some Desert Eagle has the highest calibre bullet.
Japan.
10 gauge shotgun
Probably the siege gun.
It is the dragon destroyer.
Daimyo
I don't know about Japan but for Malaysia, it's Rosmah.