Crio/Crio Plus--Carlson's Sporting Clays Choke Tubes are made from 17-4 stainless and precision machined to produce a choke tube that patterns better than standard choke tubes. These choke tubes feature a 25% larger parallel section in the choke thus throwing more consistent patterns than conventional choke tubes. Each choke tube is knurled on the end to allow for quick and easy removal. Each choke tube has the constriction laser marked on the end for easy reference. These choke tubes are backed by Carlson's lifetime warranty.