They make chamber adapter for 12/20 gauge conversion. They work fairly well and add almost no extra weight to the gun that may change how it handles. Any change like this is best if reversible and not too expensive to try. The Briley tubes are great because you can just remove them and the gun is back in its original condition but they do add about 13 ounces to the guns weight. That may make the gun handle very differently.
I had a 16 with ruined chambers which I made a set of 20 gauge reducers and installed them permanently. Had to replace the extractor and re-cut the rims lightly. Ended up with a over bored 20 which is great on clay targets and pass shooting doves late in the season.
