Each gauge in the red label is built on its own frame. Therefore, a 20 gauge barrel wont work on a 12 gauge frame. I would suggest purchasing a Beretta if you want multiple gauges on a single frame. Their 20/28 gauge silver pigeon combo is a beauty. Stoeger (AKA CRAP) is the only one that I know that does a 12/20 combo...but im sure others will know more.
Forrest
Forrest