It's prebuilt but there are many mods you can do to these things. The firing mechanism is a standard spring type airsoft rig but there are some mods I have read about using CO2 cartridges with Aluminum Airsoft BB's. I think it will require upgrading the barrell from stock to a brass airsoft barrell and then modifying the firing system.
Here is the thread I have been reading on it:
http://www.rcuniverse.com/forum/m_5403595/mpage_1/key_ultimate%2Cairsoft/tm.htm