Every weapon has to have a gunspec file.
What I do is I pick an FPSC ready weapon (files\gamecore\guns)
and I use that as the guide for my new weapon's gunspec.
If you search the models and media forum for Weapon Creator you should be able to find it.
Weapon Creator only imports your gun into FPSC...quite well at that.
You still have to make,texture and animate your weapon elsewhere.
While I did make the tutorial on how to animate a weapon I animated a handgun so you might have to adjust the animations for a rifle.That's why I use a gunspec from a similar weapon.