This is cool stuff. Can you /only/ use lego parts or could you throw in a rubber band or two? A rubber-band bolt gun (which could load itself mechanically using an electric motor) would make a fearsome weapon.

I actually was using some rubber bands at one point for bumpers on the front of our treads. There weren't any restrictions other than not using the kiddy-programming environment that comes with Lego projects, but some of our competitors were asked to cut off the weights that they had duct taped under their bot after the first couple rounds. I've built some cool Lego bolt/pellet guns, but mechanical reloading is a huge pain, and I wasn't convinced the shots would actually knock the enemy bots off course (or cause structural damage).

You're thinking of a projectile weapon. What I had in mind was a captive bolt pistol, like a cattle gun, that fires a bolt very fast and then retracts it so it can be fired again:

http://en.wikipedia.org/wiki/Captive_bolt_pistol

High-tension rubber bands or a strong spring can serve as your firing charge, and an electric motor can pull the bolt back into a cocked position after every firing.

Ah, interesting. The mechanical reloading mechanism still has the same design challenges as for a projectile weapon though.