The Odessa shims are used in conjunction with the Odessa piston and Booster spring to adjust tension of the spring. The purpose of adjusting tension of your booster spring is to allow for better cycling of your slide on a tilting barrel handgun. You can use zero, one, or two shims to experiment with and adopt one of three different levels of spring tension. Dead Air Silencers Dead Air Odessa Booster Shims 2pk DA015