If you are really interested in shooting both 12 and 20 and only have one gun, you can do this. Browning makes shot shell adapters that allow you to shoot 20 in a 12 with these drop in short tubes. I have them and use them in my O/U and are really neat. You can only use them in break open guns, single shots, O/U's and SxS's. They are not really expensive, maybe $30 for 2.
They look like black steel shot hulls. All you have to do after shooting them is to pop them out and pop out the spent hull and drop in a fresh round.