The S&W 15-22 magazine will work great with the Catch22 to provide true last-round-hold-open functionality. The only issue with the 15-22 magazines is that they are shorter (front and back length) than a standard AR magazine.
To fix this, the AR Catch22 clip-on magazine shim can clip right onto your lower for a quick and easy way to shim the magazine and solve this issue. The clip is easy to install and remove and it allows you to use your standard 15-22 magazines without any modification.
One adapter is needed per lower.