I just read an article in the Japanese paper The Yomiuri Shimbun about an air-soft dealer who was arrested for upgrading guns. 'Capable of wounding or killing' the paper described the converted guns. Still six millimeter I suppose, but firing a metal pellet. He advertised on the internet. The shops name was 'Saunders'. Maybe a Google search would turn up the pages still.

There are also occasional articles about people converting various replica guns so as to be able to fire real bullets. There seems to be a fairly significant underground gun culture here, willing to do almost anything to get their hands on a 'real' gun. A number of my male aquaintences admitted to wanting one of the upgraded air-soft guns.