Bridgestone GC Midsize 4-PW with Graphite Shafts : The Bridgestone GC iron set features Gravity Chamber technology to redistribute inefficient weight through an intricate design process that precisely positions two gravity disks for an optimum CG ( center of gravity ) throughout the set. Midsize head design combining perfect amounts of control and forgiveness Gravity Disks are progressively placed through the set for ideal performance and feel High MOI ( moment of inertia ) reduces twisting at impact Versatile sole design gives players more forgiveness in all turf conditions New scoreline design increases spin and control Soft, 431 stainless steel heads