Until now there has not been a successful means to perform a last shot bolt hold-open function in the Ruger 10/22. The reason for this is due to the rotary design of the 10/22 magazines. Unlike other semi-auto firearms with a bolt hold-open function, the 10/22 magazines have no tripping mechanism built into the magazine follower. The CST Auto-Bolt Stop with its patented “Passive Activation” function overcomes this by eliminating the need to interact with the magazine follower. Instead, the mechanism interacts directly with a cartridge in the magazine. If a cartridge is present in the magazine, then the mechanism allows the free movement of the bolt's forward movement. Once there is an absence of a cartridge, the mechanism rotates to block the forward travel of the bolt. The magazine or the magazine follower is irrelevant to the bolt stop's function. This allows the CST Auto-Bolt Stop to work with or without an empty magazine in place, making it arguably the safest bolt stop design in the world. This product comes with one modified factory magazine. Additional magazines may be purchased.