When a standard AR-15 is converted to .22lr through a conversion bolt or replacement upper, the bolt catch stops working. The Catch22 V2 slides over a stock AR-15 bolt catch to work with most common .22lr conversions and give you bolt catch functionality again. To install, just slide the Catch22 over the stock bolt catch and you're ready to go. This allows you to easily swap between calibers without the need for any tools. The bolt catch is compatible with CMMG, RTB, Spike's Tactical, Tactical Solutions dedicated upper receivers and conversion bolts, as well as similar Ceiner kit conversion bolts. For the bolt catch to provide true last-round-hold-open, the magazine that is used needs to have a follower that sticks out of the back and will engage the bolt catch after the last round. Because of this, different magazines will offer different last round functionality. - CMMG / Black Dog ""Standard"" (X-form, Sonic Weld, etc.): These magazines have a follower that sticks up after the last round is fired, but it doesn't reach out of thee back which is required to hit the bolt catch. The bolt catch will need to be manually engaged to hold the bolt back after the magazine is removed. - S&W 15-22: These magazines have a follower that can hit the bolt catch consistently after the last round is fired and are the best way to get last-round-hold-open functionality. These magazines need to be shimmed using a Catch22 clip-on adapter to take up the slack in the magwell as they are more slim than a standard AR magazine.