The thing is is that there IS no source code for SP, and there never will be. They just aren't releasing it. Weapons.dat will allow you to modify basic functionality of the weapon, and combined with reskinning, modeling and new sounds, you should be able to get some decent results. For example, that clone trooper blaster rifle thing, you could definatly do that by replacing the Repeter or something, but the secondary fire would still be the same thing, a grenade. You couldn't make it into something else, like a laser pointer, for example.
Bassoon, n. A brazen instrument into which a fool blows out his brains.