The Eemann Tech Progressive Recoil Spring for 1911/2011 pistols and their clones with 5" barrel length. Progressive Recoil Spring differs from the common spring by the fact that the distance between coils keeps reducing starting from the middle of the spring till the last coil. While getting pressure the spring starts functioning in two phases: • 1st: The first half of the spring with identical distance between coils starts compressing from the beginning. • 2nd: After the first half has been fully shrunk the second half with reduced coil distance starts its compressing as well. Each coil increases the spring compression. Full compression provides maximum resistance and reduces recoil impulse. Every Eemann Tech Recoil spring has its own unique colour that allows visual identification of the power value throughout its whole service life period. All springs are painted with polymer heat treated paint which remains flexible throughout the whole period of service. The paint does not ruin or dye handgun from inside. Table of colors: • 6 lbs - DARK BLUE • 7 lbs - DARK GREEN • 8 lbs - GREY • 9 lbs - BROWN • 10 lbs - MAGENTA • 11 lbs - GREEN • 12 lbs - BLUE • 13 lbs - WHITE • 14 lbs - RED • 15 lbs - ORANGE • 16 lbs - ROSE • 17 lbs - YELLOW • 18 lbs - BLACK Note: The Eemann Tech Progressive Recoil Spring fits: • STI 1911/2011 standard 5" Factory Steel Guide Rod • STI 1911/2011 5" Recoil Master system