Projectile Types

I haven't done anything with addons, lots of scripting though. I was just wondering if anyone could give me a 3 second run down on something like taking a normal already ingame weapon like the M107, creating a copy that shoots something silly like GBU 12's thats full auto.

Always thought that would be a bit of fun with a bunch of friends in a server. I imagine this would be just a matter of copy/pasting the current weapon, changing the classname and then having a config somewhere that declares the projectile?