If not mistaken they have a proprietary blend of a coating, Lubalox or something like that.

Description from their website:

http://www.winchester.com/products/c...N0ZXI=&type=32

Supreme® Ballistic Silvertip®: Solid based boattail design delivers excellent long range accuracy. In varmint calibers, the Ballistic plastic polycarbonate Silvertip® bullet initiates rapid fragmentation.