The CST 2nd GEN Auto Bolt Stop was designed to work with the Grand Masters Ruger 10/22 Match Bolt to give you the last shot hold open feature in your RugerA(R) 10/22A(R). The Auto Bolt Stop when installed uses standard, unmodified magazines. The CST 2nd GEN Auto Bolt Stop will ONLY work with the Grand Masters, and will not function with the factory original or aftermarket bolts. Mfg: Power Custom