The CRL Glass Sling Shot was designed to break out the center of glass circles that have been scored and run. After extensive testing, a special size and weight steel ball was selected for its outstanding performance. The Glass Sling Shot Kit consists of a flat ring, steel ball, twenty rubber bands and a threading wire. The Replacement Kit consists of two steel balls and 20 rubber bands. You will be amazed at how efficiently and quickly the Glass Sling Shot works to break out the center of glass circles.