Go to Object Editor, the Abilities tab, search for "Sphere" (Ctrl+F), set "Art - Target" to your custom model weapon, delete the Art - Missile Art, the Art - Target Attachment Point1/2/3" and go to "Art - Target attachment Point 1" and "Add string" = "left", press ok, and then "Add string" again, placing "right"/"left", up to which hand you want the model attached to. Then, Art - Target attachments, delete "3" and set it to "1". Add the ability to your unit and all set. Don't forget it's a unit ability!