AR-15 rifles that have been converted to shoot .22lr are a great and inexpensive way to shoot. Unfortunately, when a standard AR-15 is converted to .22lr through a conversion bolt or replacement upper, the bolt catch stops working.
Magazines that hold the bolt open after the last round are a good stopgap but it doesn't offer the same fucntionality as a standard AR and most ranges won't let you leave an empty magazine in your rifle in order to keep the bolt open and show that it's clear.
This is where the Catch22 comes in. The Catch22 is a direct replacement for your standard bolt catch and will work with most common .22lr conversions.
The bolt catch is compatible with Spike's Tactical, CMMG, and Tactical Solutions dedicated upper receivers and conversion bolts, as well as Ceiner kit conversion bolts.
Please see Magazine Compatability for the list of magazines that the Catch22 will work with and how they function