I run 16g in the Moggy and was having a terrible time making the whole 50 rounds on one capsule. What finally made things better was upgrading to high quality bb's which cut down on double feeds and improved things all around. I can run 12 or 16 so I'll do a comparison test this week and publish the results.
I used 12g for testing and tweaking and 16g for battle. With the 12g I was getting a full 50 round load through but not much more. The 16g gave a nice reserve to compensate for my more hectic in battle triggering. Hope that helps.