The Maxim Defense Buffer Spring Kit is designed to work with the Maxim Defense CQB Pistol Brace or CQB Gen 7 Stock, which are featured on the Daniel Defense PDW Pistol (brace) or PDW short-barreled rifle (stock).  This kit comes with two replacement springs.  The red spring is to be used when running your Daniel Defense PDW unsuppressed while utilizing subsonic ammunition.  The silver spring, or standard spring, is meant to be used for unsuppressed fire with supersonic ammo or suppressed fire with either subsonic or supersonic ammo.�