I'm not sure of his situation, but itis usually OK to use FF for a bow that is compatable. I'm not sure about the Viper, what did the paperwork say? If it does not specifiaclly state it is FF compatable, then try to e-mail or call AIM/SAMICK and ask. If there is any question, I'd go with Dacron. FWIW, myfavorite material is DF-97 for FF compatable bows. Chad is the expert though, and I can vough for his custom strings quality.