Volvik 4-Piece Metal Golf Balls Bi #83 2-Cover and 2-Core (2+2) Box of 12 Discounted Volvik 4 Piece Bismuth Metal Golf Balls 2-Cover & 2-Core (2 PLUS 2) Volvik 4 Piece Bismuth Metal Expansion during flight created energy that can extend distance Organic material expands in volume by 3-3.5% during flight.