The highest quality recoil springs from ISMI offered in 13lbs, 15lbs, 17lbs, and 20lbs. Will work with our Gen 3 Glock guide rods to tune your recoil system to the load and the feel you prefer.  These work with Glock Large frame Models(17/20/21/22/31/34/35). 17lbs is Factory weight for 9mm.