The 6mm/223 has been around a long time, and there's even factory ammuniton madeFrankenmauser, the 6x45 is a round that has been on my mind. I even made a round by neck in up a .223 to 6mm and putting a bullet in it just so I can look at it. Have you shot it or still building it.
Bob Milek liked it in his Contenders:
http://en.wikipedia.org/wiki/6×45mm