Did you create a charset of the person with the gun?
Notice that the demo for that script comes with a character named Ark. Their is also ark_knife ark_hangun, etc. To equip that new weapon you've made, you gotta make a charset of the person with that gun. But since its a shotgun you could just make a copy of the Ark_Shotgun and rename it too whatever the name of the weapons is ex) (Ark)_(Weapon name). *Ark is the name of the charset, *Weapon name is the name of the weapon. I'll look into this some more for you.