Well, if you check out the guy's FAQ, you can find out a lot about the unit. Nightmare was pretty close in the general overall description of how the gun works. He doesn't mention using an off-the-shelf motion tracking webcam, system though. He just states that the camera passes the image data on to a C# (eww!) program that he wrote which then controls the servos.
Also, I would imagine that at 26, the guy probably already has a job somewhere...
That said, I've seen similar devices before. This guy just succeeded in getting better recognition than any of the other motion tracking devices I've seen before. (He's got a good video.

)
As for the paintball sniper rifle idea, a flatline barrel will net you a little more range, but we're still not talking about *that* far. Actually, while I've never tested it, seriously cranking up the CO2 pressure on the gun will probably do a great job of increasing the range as well, but every paintball field I've played on requires you to clock your gun in and adjust the pressure to meet their guidelines. Also, with an "over clocked" gun, you run more risk of actual injury to close range targets -- something you don't want to do when you're just out to have some fun.
That said, I seem to remember an episode of Future Weapons where they demonstrated military sentry guns. If they're not in place, I'm sure there are very good reasons for it, not the least of which probably includes general safety. A machine is only as smart as its programming. A person is much more able to identify a threat and can be much more flexible in his reactions.