what part does... what?
There is a neat part called the switch, it looks like a vertebrae and lifts spent rounds into the ejection chute.
Most people who buy the FS2000 collect guns, and have the FS just for fun and bragging rights. I'd say that most people who buy it actually shoot it, but not that often, though there are some guys who really like it, and there are people who compete in 3-gun with it.