That's the one. There's another scene where she sprouts a multibarreled gatling gun in the same fashion and kills a guy who looks like an extremely low-budget Zangeif with it. At least, I hope they're from the same movie, because otherwise that means there are two separate Japanese live-action franchises where bashful schoolgirls produce lethal weaponry from their posteriors.
Go to Youtube and search 'Japanese WTF machine gun butt girl' if you dare, for video evidence.