Different magazines and followers will have different amounts of functionality with the Catch22.
Here is an overview of the most common magazines and how they'll work.
If you have a question about a magazine not shown here, please drop us a line at firstname.lastname@example.org.
Black Dog Machine Gen III magazine
Contrary to what is said on the BDM site, this magazine will NOT offer a last round hold open like a standard AR. The spring is too weak to engage the catch and the follower doesn't extend enough to push the catch into position. You can still manually engage the catch to hold the bolt open.
Black Dog Machine X Form magazine, CMMG 22ARC, and equivalents from other vendors
Because there is no extended follower to hit the bolt catch, the bolt catch won't be engaged automatically. The red part of the follower will hold the bolt open when the last round is fired but the bolt will drop when the magazine is removed. You can still manually engage the bolt catch to hold the bolt open.
Videos of Magazine Functionality
S&W 15-22 Magazine with Catch22 magazine shim
Black Dog Machine - Gen III Magazine
Standard .22lr magazine (BDM, CMMG, etc.) with extended follower
Black Dog Machine old magazine (no extended follower)