Engineered for extreme reliability, Sprinco's extractor springs use heat treated, cryo-processed chrome silicon and a specialized moly-disulfide plating to deliver consistent performance under high-round counts and harsh conditions. The extractor springs from Sprinco USA undergo heat treatment, stress relief and cryogenic processing to enhance durability They are coated with molybdenum disulfide to optimize performance The 5-coil spring is optimal for all 5.56x45mm barrel lengths, however, it may exert excessive force on barrels measuring 18 inches or longer or on bolts with very tight tolerances Conversely, the 4-coil spring is advised for use with longer barrels and when reloading brass The extractor springs are engineered to endure temperatures up to 750 deg F and are intended to be the sole springs required for the lifespan of your bolt When utilizing the 5-coil version, D-Fender rings are unnecessary