Recoil Calibration Packs are offered to allow the shooter a choice of recoil springs to calibrate the recoil function when the correct spring rating is not known. The shooter starts with the strongest spring and works down until the desired recoil function is obtained. Each Pak contains several recoil springs and offers a savings over buying them individually, including 18, 19, 20 and 22 springs. Three power firing pin/striker springs are also included. DISCLAIMER: This product is not manufactured, authorized, endorsed, or warranted by GLOCK. GLOCK does not warrant or represent that this product is compatible with GLOCK pistols.