The combination of limited chamber space and the large tube inside diameter needed for brush clearance eliminates the possibility of any type of chamber sealing o-ring. We have incorporated a front brass bushing, which centers the guide in the barrel extension and butts up against the chamber opening. The large o-ring on the back end locks into the bolt opening at the rear of the receiver to reduce longitudinal movement of the guide during the cleaning process. Made from Delrin, Brass, and Anodized Aluminum For use with any J. Dewey Cleaning Rods Mfg: Dewey SPECS: Aluminum, anodized. Delrin handle. Rubber O-rings. Approximately 9" (22.9cm) long. Includes spare O-rings and O-ring installation tool. AR-15/M16 model fits rifles chambered in .223/5.56mm and .204 Ruger.