I would disagree ( resectfully of course) unless the barrel is specificaly stamped "20 ga".
If it isn't labeled, I would slug the bore to find out exactly what "caliber" it is. The shotgun book I have says that 20 ga hase a bore at the forcing cone of .615. Anything other than that exact measurement and you have a smoothbore rifle of that (exact measurement) caliber.
If you want to go further before having it verified as a ???caliber smoothbore, if you can, or have a friend who can make up a cerosafe casting of the chamber and have a brass cut from solid bar to fit and make a slug to fit. I doesn't matter if it is simillar to a 20ga, lots of other calibers that are on the antique list have simillar cartriges (410 shotgun will not fit perfect but will fire in numerous old rifle chambers).
All of the above would be more relevant and worth the effort in the old registratiion days but as it stands now if you are out in the bush with the gun, loaded with a 20ga round and are stopped by LEO, I would want to have a PAL in my pocket.
If you are carrying it with the above mentioned manufactured bullet round in it, it would be hard for them to prove it is anything other than what you would say it is,a smooth bore rifle. Do no not mention "20 ga" in any conversation.