No one will make a full length tube for 16 in a 12. They are too thin. You can get chamber inserts for a 12/16 but why?
They work fine but why do you want them? You are going from the 12 with the most wide range of shot size, weight and powder selection to one of the least varied shell selections. Sad to say that you can duplicate anything the 16 does by loading down a 12 and selection is very good and prices are mostly cheaper. So no new loads, no reduced prices and no real pattern advantage. Might be a fun fall back if your back up gun to your 16 is a 12 and you only want to carry one type of shells. But beyond that 12/16 chambermates are about as useful as tits on a b...... . Well you know.
