NAP Thunderball Magnetic Nock System. Finally and easier way to nock an arrow. The all new ThunderBall installs in minutes and eliminates accuracy robbing string torque so you shoot consistently tighter groups. And, Thunderball allows infinite vane indexing by simply rotating the arrow while itÆs nocked. Improves tuning on bows with nock travel too. The Thunderball magnetic nock system fits Easton Super Uni and most other popular carbon sizes. Starter Kit includes 6 Nocks and 1 Thunderball.