Hi, I also replied in a thread:
http://www.airsoftcanada.com/showthread.php?t=139981
I also tried ARES/Star's QD mount as well. It is too long and its end part is too big. Might need to use a steel file to re-shape the end of the mount to fit with GG FNC's sight.
IMAG1665.jpg
This is the way I did for stormwerks mount. Find a small piece of metal which can fit into the gap. The metal needs to be strong to be stuck into the gap between the mount and sight.
IMAG1714.jpg
IMAG1716.jpg