I have one similar to the one from Jungle toy.
The gear box is non upgradable, except for the normal mini mods (streching springs, more battery, etc.)Seems to work ok for me and my kids in back yard CQB.
I plan on getting one of the jungle toy ones for my kids, even though the one I got from Shorty USA has a different barrel. (longer but vented in the middle.)
here is a link for the review of the one I got.
http://www.airsoftretreat.com/forums/to ... C_ID=52019When I get the one from Jungle toy I will try to take some pics to show the differences.