I like 1680 .. I fooled around with a lot of other powerders and I found I can load 1680 subsonic and keep the gas port in the carbine position and not change the buffers or the springs. It cycles and locks the bolt back on the last round.
With the suppressor on it works even better.
http://www.youtube.com/watch?v=gn6sXcfTwTk
http://www.youtube.com/watch?v=ilNf6OiHd5c