Interesting. For reasons known only to themselves, Auto-Ordnance started making the model 27 semi-automatic carbine they used a slightly different magazine catch. This would not work with the full auto magazines, which had a perfectly round hole for the catch to lock into. This new catch required an oval shaped hole. Machine gun on the right, semi-auto carbine on the left. You can either buy a machine gun magazine for about $20 and modify it with a dremel, or buy a pre-modified one from Auto-Ordnance for $75. Or replace the magazine catch on the semi-automatic carbine with a catch from a machine gun. All of the ones that I have seen that have been modified to work with a semi-auto carbine have been 30 rounders. To my knowledge Auto-Ordnance does not sell 20 round magazines. The magazine in your picture is a 20 rounder.