Redding .366 Bushing Tempered Steel fits Redding Competition and S-type sizer dies Heat treated steel neck sizing bushing fits Redding Competition and S-type sizer dies. The sizing diameters are hand-polished with a surface hardness of Rc 60-62 to reduce sizing effort. To determine your bushing size, measure the outside diameter of your cartridge neck with a bullet seated. Subtract .001" from that measurement to get your bushing size. Redding Neck Sizing Bushings have a external dimension of 1/2" OD X 3/8" long. They are available in .001" size increments. By selecting the correct bushing, you are then able to provide just the right amount of neck tension to properly hold the bullet without excessive resizing. These are clearly marked with the size I.D. Be sure to visit our Redding Home Page for great reloading dies, tools, and bargains!