I did run across a site call gunadapters.com that have 12 to 20 adapters. I not sure how it would pattern with a 20 ga shell. The chamber would be supported for the shell but the shot would be going down a 12 barrel and then thru a screw in choke. Some trail and error with chokes and you may find something. I'm planning on it being a defensive type weapon so running 12 or 20 ga reduced recoil buckshot it may do ok.