3155R Instead of the usual hexagonal core, the Fender® Classic Core strings feature a round core that forms the basis for a sound and feel typically found only in well-played strings. Right after installation, you get a round, deep tone with smooth highs and a flexible string that makes extreme bending truly enjoyable. Pure Nickel Bullet Ends. Gauges: .010, .013, .017, .026, .036, .046.