Your journey starts somewhere here Onsweapon class. You should look at the specific vehicle to see how its weapon classes fire things. In the weapon classes you will want to look for things that deal with ProjectileClass/AltFireProjectileClass etc...
In the future you should try to give more information, such as what you have tried, what you hope to accomplish, and a "please" and a "thankyou" wouldn't hurt!
the ONSWeapon class is the base for all weapons (Tank Gun, Raptor Gun, etc).
If you want to change a vehicle to shoot things like Tank Shells look in the Default Properties of the Goliath and find something like "FireMode" or something like that and copy it over to the vehicle you want to change.