Added weight is almost nothing, a couple ounces per chamber or less and perfectly place between your hands. I did this with a Fox 16 to 20 project. The chambers on the 16 were bad, real bad. Stored in a closet for 40 years with two paper shells in it. The shells looked like a corncob. I knew sleeving them would restore them to function, but decided to go down a gauge to make a long barreled, tight choked 20 gauge, for late season Dove. Plus I liked the idea of a over bored 20 which is what I ended up with. If you look at the bore differences you just end up with a 16 which shoots 2 3/4" shells perfectly. A lot like the chamberless guns. The chamber size was the bore size. As I recall the chamber size for the 16 is .732. With plastic wads sealing will be easy. a 16 bore is .662-.670, the 12 should be about .729. The plastic wad should seal difference. Pressures matter and there are tons of low pressure 16 gauge load information. Pressures with the chamber sleeve should be fine, it is the barrels I want to keep within reason. Sounds like a fun project.
