Eemann Tech Recoil Springs Calibration Pack "OPEN MAJOR" for 1911/2011 pistols and their clones with 5" barrel length. Calibration Pack "OPEN MAJOR" contains Progressive Recoil Springs from 6 lbs. till 9 lbs. - mostly used in "OPEN (MAJOR)" 1911/2011 pistols classified by IPSC rules. “OPEN MAJOR” pack contains: • ET-131126 / 6 lbs - 1 pc. • ET-131127 / 7 lbs - 1 pc. • ET-131128 / 8 lbs - 1 pc. • ET-131129 / 9 lbs - 1 pc. 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 color that allows visual identifying the power value throughout its whole service life period. All springs painted in polymer heat treated paint which remains flexible throughout the whole period of service. The paint does not ruin or dye handgun from inside. Note: • Installation of the progressive recoil spring for 1911/2011 by certified gunsmith is recommended.