Truthfully, what you have asked to learn to do is something that I knew WAY before i started scripting. You just have to enter something in the console - not that hard.
weaponImage.projectile="projectilehere";
Insert your weapon at weapon.
Insert your projectile at projectilehere (I.E. "RocketLauncherProjectile").
Enter it to the console.
Honestly, I don't think it's something to impress a cousin with.